Thanks for the help, guys!
I wish I knew why it blew the head gasket. I bought the bus as-is, after it had been literally sitting in a field for 30 years. The seller got it running, and I drove it to *my* field about 40 miles away. On the way it started losing power and blowing steam, which I kept topped up. Hoping the heads aren't warped, but we won't really know until we get them off.