First off a 4l80e can handle (more) power than a th400. It has a larger main shaft, larger clutch packs. They feel right at home with 900 hp.
You should never lock up your converter with anymore than a 1/4 throttle, or you will smoke the converter.
The (e) in 4l80e means eletronic, you must have a computer to controll it. I use a fast system computer. And you must make a map to controll shift points, just like a fuel map with a aem system. I also run a manual mode, with paddel shifters.
Th400 is very old tech. You need over drive and a lock up converter, to drive on the street. Th400 is good for drag racing.