Intermittent heat issues 225 slant

My 63 dart has the same engine in it and my water pump has 4 hoses coming off of it. One is a pre-thermostat by pass, one for the lower radiator hose, and 2 for the heater core. With this setup the thermostat is irrelevent. I would first test the heater core by disconnecting the hoses from wherever they are connected and run water through it from a garden hose. If the water doesn't flow freely you have a clog in the heater core. I have run into this situation in many different cars and it is usually caused by either significant rust debris or more often someone by someone dumping half a dozen tubes of aluma-seal into the radiator which then gets clogged in the heater core. there should be virtually no resistance in a unclogged radiator or heater core.

I've gotten pretty good success by holding the garden hose and the heater hose together and building up tons of pressure and eventually blowing the clog out. If you really can't get it out then your next bet is to replace the heater core.

lmk if that doesn't work and I'll keep trouble shooting with you, pics would make it real easy. Mine also heats up in stop and go and on hills so i'm going to be installing a additional electric pusher fan on it this weekend which i hope will help.