I have had problems with 650 head gaskets continuously blowing, what worked for me in the end was using solid copper ones with O-rings in the oil ways, there is less of a joint face on a 650 though.
As your head has been worked on is it possible that it has also been skimmed? The head could be just resting on the spigot in the barrels and not fully tightening on the gasket giving an oil path into the engine, although I would expect it to make its way outside the engine in this circumstance rather than inside.
Possibly due to an oiling problem?why on earth did AMC vary them?
