Heater issues in a 37 Special.

Need to have some suggestions on what might be ailing my old 37, I think it must be the water pump. Here's what I have and what I have done.

Put a heat and a/c combination unit in and a/c works great but heater gives a puff of hot and then goes cold.

I run two hoses from my Ranger pickup over to the hoses going into the 37 heater and it blew hot air at all fan speeds.

Engine is a 52 263 straight eight with a 195 thermostat.

Pulled (pressure) heater hose off, plugged heater and held hose over radiator fill; had a full stream running back into the radiator.

Heats fine hooked to the Ranger but won't heat from the straight eight.

Checked coolant temp with thermometer and it's right on 190.

Ideas please

Thank you;

Evan

Return hose from heater plugged??

Ben

Heater valve closed...?

Walked the Decatur swap meet with Ben or at least as far as two 78 year olds WANT to walk and discussed this further. He remembered that a 52 engine had a plate that diverts coolant to the heater outlet and he'd had a water pump disassembled that had sludge built up there. I'll check this out. I bypassed the shut off valve and blew through the return hose---instant geyser out of filler neck but thank you for your suggestions.