Tranny question
 

So i have been reading quite about rebuilding trannys and everyone says just a torque converter & Output Shaft. So do i need to buy a whole kit or will those two work just fine? Right now i am pushing APX 400hp and 700lb of torqe and want to add more later on. I do not like doing things twice this is why i ask if i should just buy those two things or those two things and a tranny kit what do you guys think?