Welcome to the forum
Unless it is built right a 700R4 will not hold up behind a big block. Also you need to be absolutely sure that the TV cable is connected correctly with the correct geometry and that the link pressures are correct. Otherwise you will burn up another trans shortly.
By the way, it may say Apache 20 on the hood, but your truck has K10 running gear. There is a good chance that the cab VIN and frame VIN do not match.