Sorry to hear that Nick, but if it is a head gasket you will need to change it, not a bad job on an old pushrod engine. Running it with the gasket blown may well burn away the block or head to the point where they need skimming as it sounds like exhaust gases are leaking but not yet reached a waterway which would pressurise the cooling system.
You need to take the head off, check it with a straight edge and get it skimmed if it is not completely flat. Align the head with the manifolds, torque the head down, run it until it is hot then re-torque and re-set the valve clearances.