I would say safely 700. Remember the old Hemi SS cudas and darts basically had a similar tranny to what you have and made at least 500 hp stock then got beefed up and were probably pushing 700 back in the day when billet this and that wasn't available.

An aftermarket steel drum might be good but I like the aluminum ones better for that much power because of the RPM that thing sees(quite a bit more than the engine RPM by the way).