A 3.1 or 3.4 is good, but the 3.5 is better 8)
Keep your eyes peeled for wrecked 04 Malibu's! Its based off teh same 60* V6 as your 2.8, but it makes 200/210hp with 220/235ftlbs. It would drop right in with minimal problems...
or you could wait till GM releases the 3.9 60* V6.
245-250hp.
Didnt the first GTP's have a supercharged 3.4?