351 Top End/reliability rebuild

Look for traces of oil down the back of the heads to the bell housing like HW said. If you see that it probably isn't the rear main. If it isn't the rear main it would be way easier to leave the motor in it and plop on fresh heads.

If you are going to pull the motor and transmission, I'd have to ask myself how much I'd not want to do that again. If the answer was--somewhere north of Never Again, and even like Chinese girls say "long time", I'd consider:

Rebuild transmission-- kits are cheap, replace oil pump and damper plate at the same time

Replace valve guides, rocker arms, stem seals and of course the usual minimum 3 angle valve job....

I'd r do all gaskets and seals....i mean everything. Gasket set and a few hours time....

Paint the whole engine,

Maybe do a thorough inspection on manifolds just because you can.

Id inspect each cylinder wall visible above BDC for any scuffing or obvious damage too.

All this stuff is pennies and an investment in uptime during the Summer.

Of course....the engine is out so repack the rudder and prop shaft.

Check all the cables--easy to replace while you're in the belly of the beast with nothing in the way.

Sorry if you think I'm making more work for you.

Don't throw a rear main at it until you find the leak (more likely dripping off the valve covers)