Blowing out freeze out plug after start up

Hi All,
My sons and their friends and a machine shop rebuilt my 273 for me as a surprise gift. After about 8 minutes the engine blows the left back cylinder head freeze out plug, twice. I.m sure an overpressure situation.
The thermostat is out, top radiator hose hot, bottom cold. Water in the oil. Any ideas what could have gone wrong during the rebuild. Pulled the left head, no smoking gun. Suspect / troubleshooting ???
I would appreciate your expert opinions big time. Thank you,
Glenn