The stock GE transmission will hold 350rwhp if you do the valve body shim mods, and bump up the line pressure. And the GTE version, wired up to work with the GE ECU and the valve body shim mods will hold 400+ easy.
I'd start by shimming your existing transmission, and run that, with a HUGE transmission cooler. You'll prolly last a while... And start saving for the GTE Aristo trans, shim and valve update it, and maybe get a torque converter changed over so it's a higher stall, and it'll take time, but it'll be fairly cheap.
I don't remember all the stuff I did to use the Aristo trans in the GE car (I had a friend walk be though it over a year ago)