i have 2 identical vehicles. same year. both had bad headgaskets. both are fixed. 1 has great heat and 1 has so-so heat. i flushed heater core of lousy car and it helped. i assume previous owner used stopleak. i think both cars had new thermostats installed. i have temp meter with long thermocouple lead i can attach to heater hose. would checking temp of both cars show me anything? or is it a flow issue? lousy car has new WP and new radiator. been driving it for couple months. its not an air in system issue. i think
comparing the temps of the heater hoses right before they go into the cab o the car can absolutely tell you if the one car is running cooler and thus less heat coming out. Definitely try this.
Then you will know if this is an under dash issue (blend door, partially blocked heater core, etc) or a under hood issue (car not getting up to temp, cooling fan issue, thermostat, air in system, etc.)
The most telling thing will be the difference in temp of the inlet and outlet hose. The more difference the less flow.
cool car sat out for 6 days. drove it today. its 30f out. balmy for mn. maybe vent temp is livable?