My XP coupe is running a C4. no issues or dramas for me. If you go to the right place they can build up a C4 to hold a fair amount of horsey's.. No chassis mods bolted in with a Castlemain Rod shop cross member.
Weekender:Xp Coupe, 302 Windors, C4, Lime green
Currently looking for an 9 inch disc rear end.
if fuel is a concern you will have to either run a tall diff ratio, say 2.77, or have a later model 4 speed auto fitted, which would be tough, but anything is possible. All three boxes you have mentioned are 3 speed and can all handle plenty of horses if built right. Whichever fits the coupe best is probably the best option.