305 sitting for years, low coolant and water in oil

[Dead Parrot]

Test the coolant for freeze point. Possible it froze at some point. Was it stored in a heated or outside place?

How much water in the oil? Was it coolant or just water? Wondering if you have 15 years of condensate accumulation. How overfull was the oil? A 1/2 full radiator usually needs about a gallon of coolant to fill.

A half full radiator might be due to a bad water pump seal and years of slow leakage. Or the radiator itself rotting out. Same for heater core.

Since you have already started and ran it, doesn't sound like there was any water in the cylinders. I would change the oil, add water, add fresh gas and start and run it. Leave the radiator cap off. See what happens.

After the run, check the fresh oil for both water and gas smell(leaking fuel pump).