curious to see ur cure cuz i have pretty much identical problem only thing in my cooling system that is not new is the heater core
pull your heater hoses off the back of the alt bracket and blow through on of them, if you can blow the coolant out of the hose your heater core is fine.
98 Buick Regal GS, F body brakes, Caddy STS wheels, tinted tails L36 bottom end, lightly ported heads, 1.95 roller rockers, headers, gen 5 N* 3.0 pulley, FSIC, 42 lb injectors, a BrandonHall rebuilt trans, DHP tuned and AEM water/Meth injection https://goo.gl/gpV5kW