If you dont have a leak down tester, take the detachable hose from the compression tester and take the valve core out of the bottom, then put that hose on your compressor and set the pressure to around 50 or 60 psi rotate each cylinder to top dead center (when both valves are closed) pulling the water neck and thermostat out would probably be a good idea to. Then screw the hose no hooked up to your compressor into the spark plug hole and you should be able to hear it hiss out of somewhere. If you look into the intake and see bubbles where the thermostat was you have a bad head gasket, hissing by the oil fill or PCV valve will be bad rings, and hissing in the carburetor or out the exhaust will be bad valves. I used this method to diagnose my 455. I crossed 2 plug wires detonated a ring land right off a piston, broke a couple piston rings and had oil barfing out my exhaust. I could hear the hissing on each of the damaged pistons and or broken rings .Also if you start the car with the water neck off and take the thermostat out, look for bubbles in the coolant, if you have a bad head gasket or cracked head it will cause the bubbling. My guess considering mileage would be to have the heads rebuilt and throw a new set of gasket on it and you should be in business. When the guy rebuilds the heads have him mill the heads a little extra so you can get a little more compression and a little more power. just make sure he knows how to mill an Olds head because there is a ratio he must cut at, meaning you mill x amount off the bottom, then you must mill x amount off of the intake side of the head. Otherwise you will have gasket mis-matches that happened to me with a Cadillac 500 engine and was a small disaster.
). His isn't as complicated but it does the same thing.
