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Yes, you may have a pressure leak, but on a big block dodge, if it were at the thermostat housing you would most likely actually see coolant escaping the sealing surfaces.
Now for the people who were telling the poster to drill another hole in the thermostat housing, or insisted he had a bad headgasket, you're all idiots.

The air pocket thing is the correct answer, but not to be addressed in any way you guys were thinking.
Any time you drain the cooling system of any coolant (especially if you drain enough to clear the thermostat on a B/RB motor), the air must be burped out of the system after refill while running.
There was a thread on this on here a year or so back, where there must have been 100 replies from people with nonproductive, or mildly destructive theroies on how to get air out of a cooling system. And when myself and one other person chimed in with the RIGHT, EASY, and CHEAP, and PROFESSIONAL way we were shot down and disregarded as fools.

I am a mechanic for a living and have been for many years, and this is what ALL of us use, and its the ONLY way to fly https://www.matcotools.com/Catalog/toolcatalog.jsp?cattype=T&cat=2125&page=15&#62923
Matco tools Part #SFF2A Cooling system burper kit. $30.
You seal the funnel on to the top of the radiator and fill the system through it. Then, when it takes all it is going to take, fill the funnel halfway full and leave its plug out. Fire up the engine, and the system will likely start drawing coolant from the funnel. keep it half full the whole time as you can, and as the engine comes up to temperature the pressure in the system will drive the funnel to a more than half full. (this is why you never have it more than half full when cold, so as to prevent spills) when the thermostat finally opens, you will have tons of air coming out of the system up into the coolant filled funnel, and getting replaced by coolant from said funnel. Turn the heater on full temp once the thermostat opens too, to get ALL of the air out of the cooling system. This will remove all of the air from the system that there will be, and run the engine and dont be afraid to rev on it a bit to get the water pump pumpin away after the thermostat has opened to knock all remaining air out.
Right before you shut the engine down FIRMLY place the plug in the funnel, to keep in the built up cooling system pressure (this is very important) and then walk away and do not touch that plug until the upper radiator hose has collapsed on itself some (indicating the pressure has reduce, and the now sealed system is sucking back on itself). Then remove the plug, and it will draw in even more coolant, and give each radiator hose a squeeze or two to knock any air sitting in the upper hose or upper core of radiator out. Replug the funnel, then remove it from teh radiator, and reinstall the cap.

It's not rocket science, and I don't care what your daddy or neigbor or whoever told you, but just filling the radiator and letting it run, and then topping it off sometime later leaves a good 75% of the displaced air in the cooling system in there. And with the air in there, you might as well have a pressure leak, because the thermostat does not open and flow for air. It does for coolant.
Plus, chrylser B/RB engines have internal self bypassing cooling systems, so the coolant that is actually flowing without the thermostat being open is also not going right by the thermostat (like an amc V8 or dodge LA motor does).

This concludes your lesson for today kids.

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