I just bought a ford 347 stroker with almost 550 at the crank and put a tremec tko 500. it's had no problems and will hold that kind of power pretty good. the next step up is a tko 600 or a t56 six speed. late model restoration sells the TKO's at a good proce if u buy a tranny, driveshaft, crossmember, and yoke from them. around 2,000. it a real good deal. i even put nos on top of that 550hp and made 630hp and the TKO 500 holds up fine.