How much damage can be done to a 392 in a 1975 t-all if radiator fluid got into the firing chamber through a bad head gasket? The head gasket failed two years ago and the t-all has been sitting ever since.
After that length of time there's bound to be some corrosion. There's no way to really guess on this. The cylinder head has to be pulled to replace the blown gasket anyway. Once you get that off, the extent of the damage will be much more apparent. At that time, some well focused pics of the affected cylinder(s) would give the engine gurus on here plenty to jaw about.