Here's a harness for $150.
http://www.ls1tech.com/forums/lsx-parts ... cheap.html
From a GTO. I don't know if that is a Drive by Wire setup or not, which would need some modification if you were using with a cable throttle body.
Oh yeah, the truck harness will work, but there will be some customizing done to adapt a manual transmission if he's using that, instead of an automatic. Also again, the primary thing to look for is the DBW vs Cable Throttle Bodies, as there is significant difference in the program between those two setups. It can work, but requires more mods to the harness, from what I understand.