i know that the newer bonneville GXP'* have the northstar v8 engines in them and i was wondering is it possible to put a larger engine such as 350 or to keep the pontiac theme drop a 389 crate motor in. I know the tranny would have to go but mines is in need of replacement anyway.

well a 350 bolts up to the 4t60.....
http://dtcc.cz28.com/LT1build/index.htm
but that is the only thing you have goin for ya if you put a big v8 in the bonnie. It has to at least 10 grand to throw a bigger small block in there the right way
and those newer engines are drive by wire or a wiring mess for you.

you might be interested in this:
http://killerbv6.com/
They sell rwd conversion kits for the 60* 3.x liter engines. they have the same tranny bolt pattern as the 4t60 which have teh same pattern as the 3.8l and 350....

(not 100 percent on all 350'* mounting up)