All depends on how you intend to drive this 400 hp motor. The 3550 will be a little sportier but also weaker they do fail even in jeeps with stock engines. The 4500 is more of a truck tranny but a lot stronger it also has lower gearing which may help if you plan on doing any slow speed 4wheeling since your engine is going to be pretty high strung.
Overall since your saying its a 400 hp 302 I doubt you'll really have a issue with tranny strenght as that power wont be seen until hi RPM's probably around the 5-6K rpm mark and i would even expect the torque not to peak until around 3500-4000 rpm