I have a pair of 2000 mercury 500 blue mpi motors, when should they have the top ends done and what should be replaced I have 340 hrs on mine.

Valve springs

Now is a good time to do leakdown testing on them. When I checked mine at 320 I had some supsect cylinders so we had the heads done. Very little was replaced other than the valve springs and seals but the seats and valves were reconditioned. Everyone says the bottom end is good for 500 hours easy. I know of a set used in Formula 40SS that went for 738 hours and were still completely sealed. But obviously cruiser use is different from the way most are used.

Top ends should be done ASAP.

Leak Down and Springs definately. I would personally do lifters as well as a precaution.
Isky Tool Room springs are the way to go

mpi or 500efi?