[AZKen]

History of the motor would be important. Assuming and guessing, I will give my opinion.
You did not say if you have done any work on it. So assuming you did not and assuming it's got many miles on it and not knowing where you got it ,or how long you have had it, we are left with guessing. My guess is that someone let it overheat or someone let the timing be off for a long time and these actions caused BIG heat. This created BIG pressure, and heat related damage to the gasket web. Something had to give. Probably no big deal and won't happen again. I guess. Assuming no milky oil or burnt valves, problem in valve train, no avgas or racing gas was used and assuming no other visible cracks, heat scars, etc. in head or block.

Assuming proper assembly, correct gasket and proper torque was applied. All I see is that you "freshened it up"?????????