i bought an 04 6.0 from a rolled gmc denali for 750 from a guy i met at the shop i work at. it only had 18k miles. clean. he was gonna swap it into a 68 camaro but went the big block route. i ended up selling the 6.0 for 1400.00 to a guy who wanted to replace his 5.7 ls1 in a camaro.
i found a 5.3 out of a silverado. paid 100.00 for the complete motor. searched craigslist and bought a harness, ls6 oil pump, ls6 timing chain w/ gears, set of new rockers all for less than 200 (also got extra intakecomplete with injectors and fuel rail)
i used craigslist to source the 5.3.
im rebuilding it completely: new gasket set, piston rings, crank bearking, rod bearings, cam bearings.
a friend of mine in texas is porting and polishing my stock heads, getting an ls6 cam, ls6 valve springs, getting bigger valves etc.
i'll have about 1300-1700 in the motor once its said and done.. still gotta find an ls6 intake though.
good luck with your build man.