Which transmission 700r4 vs th400 . in depth ? help!?

i have a 72 c10 swb , i have a ls1 , its going to be built, 550 hp or so , i have a th400 but i need overdrive, i could build a 700r4 or should i pick a th4oo? im going to be running a carb so i dont have a computer i know that the th400 is much stronger but also takes more hp to turn oh , i get the trans built for free so how much would a kit be to handle this? the torque numbers will be similar , i may add a supercharger later as well. how much would it cost to buy the parts to build the 700r4 and wb the th400? if i build the th400 im going to add the over/underdrive. links will be appreciated thank you!