Swapping the 3.9 for another 3.9 is the most straight forward of any swap. Swapping in a 3.5 would require the ECM & BCM of the donor vehicle & a reprogram. With the 3.9 all you need is the motor from the donor car.
I did the swap in my '06 GTP. Pulled the motor out of a Montana SV6. Bought just the long block as it was cheaper ($150 vs $500) and a new gasket set. Had the whole thing done in a weekend.
Aside from the G6, look at Chevy Malibu, Impala, Uplander, Monte Carlo, Buick Terraza, Lucerne, Pontiac Montana SV6, and Saturn Relay