How big is the hole your drilling in the thermostat? You need to bleed the air at that top heater hose. A good rad cap is required to keep the coolant full any place near the top of the rad level. Once the air is purged from the system, it needs to remain sealed or the air pockets return. Any coolant leaks increases the chance for air to get back into the system.

Does the temp gauge show the motor is warming up? If it does, check the temp at the outside of the heater core with your inferred, both sides should be the same temp. If the temp varies more then 10-20 degrees, the core could be the problem. Turn off the heater fan for this test! The heater fan can cool down the heater core by several degrees in a very short time, especially if its cold. Check metal fittings if possible. Hoses do not give good readings.

Though a real temp gauge with numbers on it would be better, you can get an idea of the motor temp with your inferred at the intake manifold just below the thermostat housing. You can also check the temp of the rad core tubes at the bottom and at the top of the rad. You can see about what temp the coolant is flowing through the motor. You have to check the temp of the metal that surrounds the parts the coolant flows through for the most accurate numbers, and they will still be off a bit, but if everything is checked the same way, you can get an idea of what its doing.

I have to ask, how long is the motor running before you are expecting heat? If everything is working properly, it will take at least 10 minutes running time before the system BEGINS to produce heat, and another 10 minutes to show heat in the cab. If you have the heater fan on high during this time, you can almost double those times! Its amazing how many people have the heater fan on high and wonder why it takes so long for the ride to warm up. Are you expecting that little heater core to displace all the cold air in your cab instantly, but your not even letting it warm up first? Gene