buy a GTO or Bonny GXP.
The only FWD trans that Ive seen fitted to a V8 is the 4T80E mated to the Northstar in the caddys. That trans wont fit in the Grand Prix, only the bonneville, which is why that bonny GXP can be a V8.
IMO FWD V8 is prob out of the question. Unless you can do a lot of fabricating and make that trans fit, or make the car RWD.
GM made two prototype FWD V8 Impalas (2000+) using a northstar and an LS1. both were FWD but I think the Impala is the same size/class/body as the Bonneville and both used the 4T80E.
Jeff