Have you checked the flow thru the heater core? Are you sure the cable isn’t binding and the temp valve is opening all the way? When my temp valve started to leak, I had a 3/4 turn ball valve on the head to stop hot water from getting to the heater. The heater seemed to work much better that way, before I installed the in-line valve. It’s a pain in the a$$ to open the hood to mess with the ball valve, but this fall I may go back to it.
These vehicles were run in cold climates when new, so there’s gotta be a way to get heat.
Please share what you find to be the remedy, when you find it.......Mark