Thanks all for the tips & insights.

I too went through the entire cycle of replacements -water pump & thermostat.
I could not get any flow through the core. I COULD pass water directly from a garden hose through the core & no leaks were noticed inside. I put a ~6" section of clear plastic hose in-line with the heater core line back to the thermostat housing. This proved the no flow. I put the front end up on ramps & this got the 'air lock' out of the core & the water flowing. The heat is good.
I hope this helps someone else from all the testing & guesswork.