This would depend on a couple of things --
1) is anything coated? If not, you will be giving up a lot of heat with the headers and a high flowing exhaust, BUT you can recapture some/all of that by mounting the turbos after the muffler (don't know where your muffler sits now -- might require some reworking). You can definitely keep the headers, just get a good coating job on 'em and possibly wrap them to keep the heat (depending on how much turbine you're willing to give up for spool -- see #2)
2) When do you want it to spool up by, and what are you willing to trade away for a quick spool. For me, I went with pretty small turbines for street and the ability to switch up if I want a glory run. We're guessing I will get close to off idle boost capability with the street setup, but the change in my exhaust plans may put a wrinkle in that.
3) in my experience the stock manifolds lost their allure when stuff caught on fire during a dyno run At that point Rick@STS suggested I get them coated and have them worked over to flow better -- which priced out more than the set of coated bassani's I ended up with. I did not notice any impediment to spool time from the manifolds to the coated bassani midlengths, so if there was a loss it was too small to notice SOTP. Unfortunately, I broke before I got back to the dyno after the fire, and it's been on jackstands since...
4) again, depending on the layout -- it might be an easy thing to slip a turbo in there and connect to your existing tips. The turbos want as little back pressure after them as possible, though, so if you elect to put your muffler behind the turbo, you'll want a cut out for sure (and put the cut out as close to the turbo as possible and dump it straight to air).

I've had stg III TEA 317s from the get go, and while it allowed me to get pretty close to Parish's initial 408 numbers (I believe it was the first PT88 runs at around 14psi and 317s -- but not exactly sure anymore -- the 700ish runs) with a smaller turbo -- and one that was too small, really -- I agree with what he said: stock 317s + turbo will take you a long way -- and way deep into the range you're talking about. Knowing what I know now, I may have saved the money on the heads...

I also agree that a gts76 up front or a "GT76" from STS in the rear would put you in the 700s without much effort. The "GT70" from STS put me pretty close to that, if not there (never know thanks to the fire), but it was WORKING to get there -- too small really.

IMHO, a 408 + turbo is an easy win for 700hp -- front or rear mount, but definitely spend some time on the intercooler and run some meth. I also probably wouldn't go to the expense or hassle of twins to hit 700, unless you just want to do it -- its too easy with a single that still spools reasonable well imo.

 

as for when it kicks in... with the setup i had i wouldnt feel much untill about 4k in rpms and it would pull solid till shift point of 6800. [i just recently found out the power range on my cam was 2500-7500] i at the time didnt knwo what the rpm limt was on that cam. i found someone interesed in my headers. so that leaves my exhaust to be saved. i was thinking if i went twins i could have them matted to the exhaust. exhaust is straight tru 3". right now i would like to go turbo, have always liked them but worried sorta abotu cost. blower would be simipler but not. i have fbody accesores and the kits for ATI ar down low which i dont think will work with a trucks frame. a custom TT setup would be nice. was thinking maybe have them mounted down lowlike right after the manifolds end. a buddy just did this same setup for a TT TA. nwo what i want...

if i go twins i want as much as i can get with somewhat streetablity, would like to use full range of my gearing of 4.88s. would also like to use the rpm range i have now. 7k being the limt. just cause of gearing setup of 4th and trap speed.

it seems that if i go twins i can skimp out on heads, if i go procharger i need to lay down cash for good heads. now with properly size twins i would like to be around high 8xx/low 9xx. figure with this much power it would be basically a conversation piece that not to many woudl wnat to catch on a roll.

 

Either way -- twins or big single you can skimp on the heads most likely.

I did a custom rear twin kit with straight through 3" exhaust and stainless works headers (see the link in my sig to custom TT for mock up pics). Doing the intermediate mount thing (low mount? whatever its called these days lol) is also a good option -- some of the benefits of rear mount, but with more of the spool characteristics of front mount.

You can set up the turbo to hit at 2500 but it will fall off by 7k most likely. Or you can do the reverse and focus on the top end. I usually ignore anything north of 6.5k for a turbo setup in favor of a faster spool, and size the turbo to hit around 3k or so and pull hard to 6-6.5k (and then get a converter at 3k so anytime I floor it I'm in boost -- like slowhawk says above)

BTW whats your cam (or are you going to change it to suit the FI choice)?

 

Thunder Racing Custom Camshaft
"sTRoker" - 244/248 - .612/.615 - 112 lsa 2400-7500 RPM Power Band. Intended for engines 6.6L (402cid) and up. Lopes hard. Strong mid-range & high RPM power. Requires computer tuning, 4000+ converter on autos, 4.30's or greater depending on transmission. Works very well with nitrous and proper tuning.


that was my cam. and remember a built T56. no converters for me. what would be a good set of twins to use?. also all numbers have to be made on 93 octane.
i want a fun truck. it was fun with the 408, very driveble in SD but ive always wanted boost so now heres my time and im already stuck with a 408

 

You might want to look at a cam with little to no overlap for the turbos. Jim and I run very similar cams -- mine is a 236/230 on a 115lsa in the same lift range as yours, and I think Jims is a a 236/234 or similar. The closeness of the duration on your TR cam is promising but the LSA makes me think overlap city (which will be deterimental in a boost motor).

I'm with you on the 93 -- I turn all my numbers on pump gas (but will run meth injection since I can run that on the street too).

I'd forgotten the T56... That would be a freakin fun ride... TT 408 T56 lol.

The twins depend on the front mount vs rear mount thing. The ones I'm running are rated at 630 each at the flywheel, so would definitely support you at 700, but for 700 and no more, you could go a more standard t3/t4 -- something like precisions 5031E which has a variety of turbine sizes (.48 to .82 I think) so you can tailor the spool time to your liking. Turbine housings are pretty cheap -- ~$150 or so, you can do what I'm doing pretty easily. You might even get away with stg I T60-1s but I'm not sure where they fall out for hp.

 

well looks like i got some time to figure things out. my cam now has about 22* of overlap i believe. like stated if i go turbo i want closer to 9 at wheels. where do you think your at with your current setup

 

Planned for 1k+ to the ground with the bigger housings, but with the "street" turbine housings will be around 800 to the ground -- but thats through an 80E.

I ended up having to build the entire exhaust because the bassani's hit the 80E on the PS, so I built 3" and coated everything (plus added two 4" maggie bullets). Hopefully all that will be in the same heat range as the bassani's + 2.75 stainless pipes I had before. If it's hotter, I can run the bigger housings all the time, and get something even bigger for glory runs.

At this point though, I mainly just want to drive the damn thing and have fun lol

 

yea i just took soem time and looked over your build you ahve in your sig. yea with 80 your sucking up alot of power so with a manual ill be alot higher in the dyno ratings.
now where do you think youll honestly ET at? so do you think just run stock 317s or find a good set of ported ones? i think as time goes on youll possibly see a few Pms from me concerning size and etc. i hope for this to be teh very last time to mod this truck

 

ya. what he said

 

I think if I can get it to hook I'll hit mid to low 10's.

I'd try the stock 317s - you can pick 'em up on the cheap these days. I spend any extra money on the turbo system.

Fire free on the PMs man -- don't mind at all. I'm moving in the next week or so, though, so the near term may be a bit spotty with replies.