Q: No hot air coming from heater

The very first time it overheated I retarded the timing just a smidge because I noticed the distributor was not tight and thought it could've shaken into advance. It seemed to work, the temp immediately went down and stayed down. happened again By overheating I mean needle buried in the "H" no steam or blown hoses.

I decided to change the thermostat thinking that was sure to be the problem. I took off the housing and there was NO THERMOSTAT at all. Good grief. But now I was really stumped at why it's running hot. I installed the new thermostat, double checked I put it right way, and with a clean seal and new gasket , tightened everything back up. No change. I flushed the cooling system w/ distilled water/flushing through heater core inlet hose. No change. What should I do next? Heater core or water pump? The entire time over heating no hot air from heater at all. Cold air only.
My car has a manual transmission.