To much build up pressure in coolant system!

As an FYI, one thing that has been going on is that the large irregular hole in the front of the head gasket is actually where the head gasket has corroded out. The original does not have a hole there; it is solid there to block coolant flow. When they have been in a car for 30-40 years, they corrode through there, and allow some of the coolant to bypass the head and block; most of it should flow to the back and the up through the 2 large holes in the back of the head gasket, so that most of the coolant flows through the length of the block front-to-back and then through the head back-to-front.

If I had to guess, I'd guess that is Bar's-Leak in the overflow, as suggested. It is pretty consistent and not quite like oil in the coolant. And there does not look like any coolant got into the crankcase, judging by the appearance of the lifter galleries. Drain the oil and check what comes out first.

It'll be interesting if the shop reports head warpage. BTW, what did the 'horrible noise' sound like?