My heat sucks

Determine if coolant is reaching thermostat’s rated temperature, if not, remove and test stat in a pan of hot water; replace as necessary.

At the heater core’s end of the system:
Several conditions will cause poor cabin heat the first you have addressed by flushing the unit for good flow. This leaves the heater box with “rubber banded door” which is not necessary with a properly functioning box.

Two reasons for poor heat production come to mind; air flow through heater core is blocked by assorted trash such as leaves and rodent nests etc., and or air control flaps have lost their ability to seal off incoming cold air that mixes with heated air destined to cabin and defroster.

You need to remove the heater box, clean it out, and rebuild it with new seals & gaskets. Unfortunately Detroit Muscle does not offer rebuild kits for trucks, but that shouldn’t stop you from adapting some thin foam cut to replace the old decomposing foam on internal flaps.