GM Performance has a 364 ci (6.0L) 440HP crate engine. P/N 17802134.
It is complete from oil pan to carbureted manifold. All you need is an MSD6012
ignition to fire it and a carburetor. I think that would be a decent choice.
http://www.gmgoodwrench.com/GMPerfor...2134&engCat=ls
About $5200-5300
http://www.msdignition.com/2007/ls2controller.html