A stock turbo 400 with a brake will take 700 hp almost till the end of time. A bell, some work on the mount, and forget about it.

I have a couple of expensive 727's , and they have there charm in being super simple to work on, and they're lighter than a 400, but once you start making powe,r the relative cost starts on an upward trajectory. A stock 727 with a safe drum will take 700 no problem too, but if your plans include more, and more power in the future, use a 400. IMO