You could upgrade to a borg warner velvet drive. But, it will require new mounting holes for the engine, as the trans is different. Might be worth a call to bam marine and talk to mark. Im not sure what the merc trans can be beefed up to handle. But stock, they don't handle much.

You have to move the engines forward either 10 or 18 inches (can't remember which was correct). Depending on how you are planning to use the boat the trannys will be fine with 500hp.

I broke a few TRS lowers in front of a pair of 550 hp 502's in my old Scarab Panther. The uppers were pretty trouble free but the pinions in the lowers were less than desirable. They would last a few years but I did have 2-3 failures over the course of time that I owned the boat.

My RH trs lived 15 hrs behind a 502 mpi with a new Bob Madera valve train but it was on a heavy scarab 400, 502's really woke up with new set up