transmission questions

[C10longwb]

hey guys Im in the process of building a 350 for my 70 c10 itll probably have around 300-350 horse nothing fancy just a stock rebuild.. I have a th 350 trans that was in my truck that has been sitting for 20+ years i got the truck to drive under its own power when I first got it only about 10mph due to the carb backfireing as I gave it gas so it never got to shift but it went in drive just fine and reverse just fine no hesitation is it worth just trowing new seals in and seeing how it long it lasts? or is it a waste of money in fluids I was told I should put new front and rear seal and a dip stick oring and new filter and drive it till it blows ...there was nothing in the pan and the fluid was pink and un burnt... im on a budget with this truck and trying to spend the least amount of cash as and I was wondering if I was to get a differnt transmission what does it take to put a th700r4 behind a carbourated sbc I dont know what all it requires thanks in advance for any advice