Well, any GM 60* V6 will bolt up to the 2.5L's bellhousing. Most people will say to stay away from the 2.8L (the smallest in the 60* V6 family), and I'll agree. They have the smallest displacement and were also the first generation. These were used in Cherokees, S-10s, Camaros, and I think that's it for RWD. Then there's the 3.1L (second engine in the family), which i'm not sure you'll find in a RWD application. The 3.4Ls are probably you're best bet. These are the largest of the family and can be found under early 4th generation Camaros. Edelbrock also sells carb manifolds for the 2.8L, which might bolt to the later 3.4s (I'm not sure if they use the same heads though).
Anything else, you'll need a complete drivetrain. The 2.5L bellhousing can bolt up to a lot of engines (Jeff has a post abou tthis) but you just don't want to AX-5 behind them.