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well if your lucky it may hold together for 500-550 HP. That would be with some luck. The 5/8" input shaft is the weakest point to any high power outputs. Third gear is a common failure point when pushing these trannies.
Mine broke at about 450 HP but that was just mine. I have seen several break at the same level though. My mantra is dont fix it if it aint broken anyways.
If you're spending the kind of money to get your car up to 500-600hp, you might as well spend the money to modify the transmission/driveline as well. Hypothetically you would have already spend up to seven grand as it is, (turbo/blower kit, exhaust, etc)