"Ok, let's see what we can
"Ok, let's see what we can do. Your bearings are seizing from no oil. Sometimes bearings are put in upside down and the oil hole doesn't lube the cylinder but that doesn't appear to be the case here.
So, if you'll pardon my simplification, because you might have already done some of these things, here's what I'd do.
You have to open up the bottom of the engine again. Replace ALL bearings, yes again. They've all been run without oil. It is expensive, but needs to be done. Have the crank checked and polished. If you've seized two bearings on one journal and one on another it simply can't be clean and smooth enough to be reliable. (yes, mains and rods) When you're assembling check the clearances with (brain fart, comes in a paper sleeve with the string like substance inside. Put a small piece in the rod bearing, tighten everything up to spec then take the cap off and measure how wide the string like piece squished to, to get the bearing clearance) Put lots of assembly lube on all bearing surfaces. In the old days we just used oil or STP, the assembly lube is much better.
If you never had the camshaft out yet, you may need to replace those bearings too. They don't have nearly the pressure on them that the rods do, but they sat for at least 6 months with no lube and then have run since then with no lube. Your call as to whether to do them or take a chance.
I would replace the oil pump. Make double sure it is the right one. If it was replaced before.... are we sure it was the correct replacement? There is a pin at the top where the gear is connected to the shaft, make sure it is in place and not broken. Clean the pickup tube and screen. When you're done, you should be able to put it on grandmas thanksgiving table. (cleanliness is important) PRIME THE PUMP. Put the pickup in a clean can of oil and rotate the shaft so it picks up oil and pumps it out. Make sure when installing it, that it is engaging the cam drive. Make sure the pickup tube is in place and tight.
Remember too about burping the coolant system. There should be a petcock in the front, on top of the exhaust manifold. Open it and pour the coolant in slowly allowing air to escape. SLOWLY pour the coolant in. These engines air lock very easily.
Button everything up again, put in 12 quarts of oil and give it a spin. If I was nervous, I might even close the seawater inlet valve and crank the engine with the shutoff button pushed (or cable pulled) and let the engine pump oil without firing.
You should be ready to go after this. Remember, boating is a lot of fun. Right now it's a pain, but better days are coming. Please write back if you have more questions.
K"