change the heads and cam,lower the compression a bit.

Roller cam, Intercooler, freshen blowers, and pulley for max boost.

Currently running Whipple Inner Coolers and smaller pulleys. I am thinking about 871's. Trying to decide if its time pull the engines and go for the complete rebuild.

i did mine at 15X

refreshed bottom end rings and bearings, stock merc components,
525EFI cam
AFR CNC 305 heads - big chambers dropped compression even lower then stock
.040MLS head gaskets,
dyers 8-71 kit
eddie young EFI ontop
Holley HP as the mefi **** the bed on the dyno -
made 775hp on 4psi at 5900 un cooler

with the whipple cooler im sure you can do a super easy 8XX

At 410 hours, I think I'd just do complete refresh.

Eddie Young did good (top end only) on some 575's we had. Heads, roller cam, tuning, etc.

I went .030, cleaned head and deck, 525 roller cam(I think), Super Chiller w/slightly smaller pully, stock fuel system, stock MEFI
Cam I .610/298* E .632/306*
Press fit pins and cast iron heads
Stock oil cooler
703HP@5372 740TQ@4100
4# boost
92 octane
Ed Trihey @ Ed's Automotive assembled/dynod them
310 hours and still run the same based on top speed of boat hasn't lessened.
WOT oil pressure 38PSI@170F@5350RPM for 10 minutes+ freaked me out but 310 hours later I guess it works and it's easier on the distributor gear.