Don't kidd yourself man. This is going to be a huge undertaking. Sounds like a challenge even for the most experienced gear head. Will the engine even fit in the engine bay? G6's have a small, narrow chasiss especially up font.
No one on this site knows a thing about G6's. There is a G8 section, but its pretty thin. You're gonna need help from the G8/G6 forums more than from the 3800/W-Body guys.
Good Luck!