Technical Problem with radiator cap

Been doing this for a long time, but never ran across this. Helping a buddy with a 41 Willys. The T-stat housing was much higher than the top of the radiator, so I had him get one of those housings that have the radiator car on it, so it would bleed off air at the highest point. Bought Summit Racing's version, put it on, and it leaked. From the rad cap! That meant it was leaking from the pressure seal first, AND the cap seal as well. Tried 2 other new caps, all the same. Tried cutting out a pressure seal gasket out of 1/8" rubber, to double it up. Still leaked. Cut a second extra gasket, but was too tight to tighten the cap.
Summit was very helpful, and actually sent us a new on, the Moroso version this time.
Damn, this one leaks too! So tried cutting an extra cap gasket this time, and it worked. But I don't know if the pressure seal is going to leak or not.
More info. Used 7, 12 and 20 lb caps, no difference. Tried putting my rad pressure tester on it. Pumped like mad, but it would not hold pressure at all. It does work on another radiator, as well as testing the caps. Also, this does not occur when the engine is hot and under pressure, it happens almost immediately after starting. Below 130*. Never got it hot, as it leaks so much, to see what would happen. Also checked to see if by accident, he had put on a reverse flow water pump. Nope, casting numbers, box numbers, and flow check all say standard rotation.
Any idea of what's going on? I hate to leave the second gasket on, it may get lost at some point. And I am not sure the pressure seal is going to work, either. Unfortunately, we have to get the engine running right, broken in before being able to go any farther with testing the cooling system.

 

Did you measure the height of the neck, between the surface that the cap contacts? (the small hole in the bottom, and the top rim) Compare to other radiator necks.

 

Like Squirrel said check the measurements, it might need a different cap or you could find a housing with a threaded hole in it and use an air bleed valve.