On your mopar, converter or trans slip would affect H.P. as well as any brake drag,excessive rear end friction etc. could also be fuel supply,200 deg air entering the carb,dist got bumped when installing the engine, or a ton of other stuff. a good dyno operator should have been able to find the problem.that engine if it was an automatic trans should put down around 530/550 to the wheels so there was a problem somewhere.
Since dyno Jet drums are all weighed and the electronics calibrated at the factory to all read the same and have absolutely no user changeable adjustments they should show the same numbers going from one to the other assuming same car and same air temp, pressure etc. other chassis dynos that use a load control instead of a fixed weight will vary quite a bit.Bill