does anyone know what all would be involved in converting a 4.8 to run an LS1/LS6 topend?

 

topend? i.e. heads and intake manifold?
if that is what you are talking about, then you will have to swap over to F-body (LS1) engine accessories. To avoid having to spacer anything, I would buy an F-body harmonic balancer to do the swap. The truck harmonic balancer sticks 5/8" further out than the F-body. The balancer is what everything has to line up to, so if you retain the truck balancer you will have to spacer all of the F-body engine acc. outward 5/8".
The heads will bolt right up, but you will be dropping compression ratio with the LS1 heads.
You might just swap in a real LS1 for a little bit more cash. Use the truck balancer and engine accessory brackets and everything will line upi fine. All of the same bolt holes that the truck uses to mount things is on an LS1 block -in the correct spots too