I suppose if I were to go the route of a new heart *exotic* transplant the LFX from the Cadillac ATS & the 6T75 trans would be my choice of powerplant. I won't sneeze at a 320 hp na V6.
The 3.6 is quite an engine. The 6T70E seems to be a pretty-fair transmission, although it pisses me off that there's no serviceable filter.
But giving up a V8 in a fairly-rare car to go backwards into ordinary? Re-think this.