i thought the noise was from piston slap, and GM had a tsb out it and even offer .002 oversized pistons? everybody i know with a 6.0L and a few people with 4.8's and 5.3's has some form of tap/rattle in the motor.

 

piston slap is a cold motor noise though, this was a hot noise

 

I had noisy lifters for thousands of miles. If you're going to replace the whole engine anyway, you can just run it like that until something breaks. It might take a while though.