Any outdrive will corrode if you leave the boat in salt water. I don't know what they cost and they might not be cheap, but the plastic Volvo outdrive is the best outdrive out their now. It is tougher than aluminum, is more efficient, and has a remote oil resivor. So at least you can catch the water in the oil before a bearing is wiped out.
For five grand good luck, remember you will need new tanks, fuel lines, engine mounts, maybe new guages. Depending on the year of the boat the stringers may be getting soft, that could be a can of worms. If you went to a fuel injected 302 you would probably be doing better. It is a heavy boat for a single small block to push along.