I've got a 1999 mustang GT with the 4R70W. previous owner didn't do the fluid changes and it started acting up.

looking at either a new replacement or something upgraded.
I'm not sure what my opinions really are. I want something that could handle 600-700 horsepower because I have big plans in the coming years. for those who have experience in that department what do you all suggest? Could I stick with a 4R70W that's beefed up or do I need a whole different unit. price range currently is up...

Transmission replacement/ Upgrade