dm and Paul are right. Heads, valley pan, oil pan, pick up screen. But before I did that, I'd run it with the valve covers off and check if oil is still getting to the rockers. That will tell you if the block passages are plugged. Has the car been driven much, or has it been laid up. Is the draft tube filter clean, and the oil fill cap mesh clean, or can you fit a PCV valve (just so a search for that, several different ways). It's always a chance, but worse Y's have been brought back if the internal passages are open. Lack of maintainance, no PCV or a plugged draft tube system, and non detergent oil did that to a lot of motors. Y's are pretty tough, and you may avoid a major rebuild. You should have an oil pressure gauge, and note the pressure, also.
If you get it cleaned up and running, changing the oil and filter frequently will help. If the oil looks good, at least put a new filter on and cut the old one open. See what's rinsing out of the motor.