5.3 swap coolant problems

[mopar250]

Hey guys,
I have swapped a 5.3 into my 80 short bed and ran into a problem now that I have it running. Now that i've filled the cooling system, the engine won't take any coolant. When I start up and let it run for a while the temps on my guages get up around the 220 range and both radiator hoses are still cold. My gauge package is new but since the computer kicks the fans on at 180 or so I believe it that the engine is at 200 plus. I have a non stock radiator but i've only added about 2 gallon of antifreeze and looking around at capacities I need to be at 3.xxx. I assume its one of a couple things, either and air bubble or I possibly put the thermostat upside down? I started this whole crusade over a year ago so I don't really remember much about the thermostat other than I put a new one in. When warm I can turn the rad. cap and there is pressure in the system. Anyone have any good ideas to try?
Thanks!