Coolant issue

[08impreza]

I have an 08 impreza 2.5 n/a I just did a motor swap on it but doesn't seem like the thermostats opening. Thermostat is new. Never had this issue before on any other subaru but is it possible for a clogged heater core to cause the thermostat not open and coolant to bubble back into the funnel? If so ill try to bypass the heater core and run it again. Just asking before I replace the head gasket since I think its the exhaust coming into the radiator.