I think the first thing I would have done is put on a solid radiator cap and pressurized the whole system with about 20 psi and found out where the coolant was going before the teardown. That small block sure got toasted! Got pretty much off topic, but an interesting video.
This is who I would have suggested you talk to if I talked to you sooner. One of the best Chevy engine builders in the country and he is here Bohemia NY.
I had my 70 SS 396 starter rebuilt due to heat soak problems with the solenoid. I added a GM heat shield that should have been there. I will see if it works.