I think a further cleaning is definitely in order. Is there any value in bringing it up to temp without a cap in place? I do have another cap in the garage parts stash I can try out.

Unfortunately this radiator's filler neck is a tube extension off the tank so its impossible to watch the coolant flow by. The possibility the engine has a bad head gasket did occur to me. I might borrow a friends leakdown gauge to test that theory.