LS1 based, yes! I know for sure that the 99 6.0 is all iron. I think the newer ones have aluminum heads and the ones that are in the caddy's and the new SS are all aluminum. Not sure though! As far as the fbody and corvette thing, pretty much every thing is universal, but I think the intake manifolds might cause you a problem with the TB. The intake manifolds on the fbody and vettes are lower than the trucks. The truck intake manifold will produce more torque. Also the 99 Crank is shorter than the 2000 + motors, this will cause a problem with the newer trucks 4L60 and 4L80 trans. Brackets and FRC will need modifications.
If you want a lot more info check out www.LS1TECH.com
What kind of project are you doing?
Ben