Well, the new Bravo-1X is rated at 425 HP, and about 450-500 ft lbs. Those of us with big boats, 425+ HP, and a standard Bravo are REAL easy on our drives, as Merc realized the standard Bravo wasn't cutting it for too long under 502s and 496s. Although, there are several peeps here on the boards running moderate blower motors with B-1s and they are living. I think keeping good lube (I run synthetic TORCO racing trans lube), and just driving it sanely is just common sense.
I am sure others here will back me up on the prescription for your client:
If the 502s are in good shape internally, low hrs and such, I would say a Whipple / apiece. That will take the 502 into the 650 HP range.
If he/she wants to run the B-1s till they puke, so be it. Otherwise, money permitting, throw on two Bravo-XRs. They are made to handle that kind of HP.
Optional would be nice exhaust. Again, money permitting.
Does that help?