I suspected that the coolant system was building too much pressure. Decided to check the overflow pipe. Disconnected at filler neck on expansion tank. Blew.
Blocked. Removed rear panel in LH fender. I don’t think it has ever been removed before. Kink in overflow hose! Also, bracket holding the overflow tank has rusted to dust. Car itself minimal damage, but bracket gonzo. Guys, this is worth checking!
Did the 3-stage POR-15 process on the slightly-compromised shelf. Made a little well-drained box. A new hose and a couple of big cable ties. Good to go!