If you have an exhaust-cooling system HG leak, then stop leak will do no good. What happens is that the combustion pressure in the cylinder forces itself into the cooling system, partially filling the cooling system with exhaust gas. The gas rises to the highest point in the system, which happens to be the heater valve and heater core. If this happens, you may be able to hear a gurgling or whoosing sound when you rev the engine with the heater valve open. A cooling system pressure test can be used to check if this is happening, you look for the system pressure to rise with engine RPM. Or you can do a chemical test of the coolant looking for exhaust contamination:
I found lack of heater output was almost always one of my first signs of a HG leak on my 22RE.
You might also inspect the heater valve inside, remove the hose and make sure the valve is not clogged. I did have that problem on my old Landcruiser. The valve was almost totally clogged with deposits. A new valve and the heater output was much improved.