Possible No flow to heater core?

our 1st 67 without a/c had no water control valve and very poor heat. It was easy enough to take the hoses off the slant 6 and drape over the right fender. I flushed with garden hose until water ran clear. When I changed flow direction the water ran nasty again. Repeated this several times before water ran clear both ways. My result was toasty.
Thermostat stuck open is a possibility also.