I would have to agree with 99BonnevilleSE! The best choice (& easiest) swap would be the Series II 3800 */C & 4T65E-HD Trans pair! You would still have to change the engine & trans harnesses, as well as the ECM too.
The good news this swap should be able to handle just about anything you could dish out! :P NOTE: Even Speedyguy possible couldn't wreck this combo!
Sorry about that jab Ty!!!
I might still do this swap or maybe something even more radical like the NorthStar V8 & 4T80E swap from a 98 up Seville! I figure with THIS combo & the Bonneville'* weight I could blow the doors off anything at the track in the quarter mile!
Can you imagine! A Bonneville 4 door family sedan sleeper!
By the time I'm done the car will not look the same! Radical hood redesign with ram air scoops and custom front end work to bring out more of my "SKULL" motif!