Help!! th400 questions!!!! is their a th400 with over/underdrive with out a computer?

i have a 72 c10 ls1 engine and i need overdrive , 700r4 wont handle the hp but i neeed over drive but my engine is carbeurated so it doesnt have a computer , i need help ! its going to be used for street mostly but strip also im going to be at least 500 hp how much would a th400 with overdrive cost and how much hp could it handle or is their a kit for a 700r4 that will make it handle that much? i will get it built for free because my uncle used to build them but anyways thanks any help will be greatly appreciated