While a working heat riser is a good idea, it's very unlikely to cause the coolant to overheat. Try putting a little pressure on the radiator with a hand pump and see if it goes away quickly. If you're running the original radiator don't go over 4-5 PSI or you're likely to spring a leak in the radiator or heater core. I'm going to guess the pressure won't stay there long, and you'll probably find a blown head gasket or a cracked head, or possibly nothing worse than a stuck thermostat. If the pressure stays there for 10-15 minutes or more, pull the thermostat and do a boil test on it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!