Look for a any "G" body frame. Monte Carlo, Buick Regal, Cutlass, etc., 1981 - 88. These are a "car" chassis and you won't have to deal with the "truck" kick-up behind the cab. I would suggest buying a complete, running vehicle. That way your swap is a relatively straight forward body swap and you know what you are dealing with for replacement parts. It's also less likely to nickle and dime you to death like trying to piece something together...
There is also a growing aftermarket for these cars so suspension, engine and driveline upgrades are getting easier.
Mark