whats the easiest and cheaper way to get a strong auto capable of running around 700-800hp. either converting to a th400 auto gear box or rebuilding the supra auto gearbox?

It really matters what you want to use it for, I don't thing there is going to be a large price difference. A built th400 can be made to take more than the Toyota tranny for cheaper, but then you only have 3 gears...and you have to adapt it to the car. (not really a big deal, but still, its more parts) If it's a daily driver, I would try and build the Toyota Tranny, it might end up being a little more money in the end, but it will be WAY more street friendly. It really depends on what you want to do with the car.