From my notes (now that I'm here at work), the merc number for the 500EFI is the 849896 for the injectors. The 502Mag, 525EFI, 7.4MPI, 454EFI, 454Mag, 8.2MPI, 377Scorpion, 350Mag MPI all use the part number you listed above.

500 EFI and 525 EFI have 42 LB injectors and pressure the same 43-45 psi, this comes from Whipple Superchargers, they upgrade them to either 63 LB. Long Injectors or 65 LB. Short Injectors, beyond that they regulate the boost-a-pump to increase fuel pressure for added fuel, this should handle any fuel needs up to 1000 FT:LB, after that either injector can be milled out to 85 LB. at 43-45 psi to go beyond that, any more speed than that, the pounding the hull will take will crack the stock aluminum gas tank it's supporting.