Still Can't Find the Right Gas Cap

[DT1]

I could really use some help here. I have a 72 C10 with a vapor recovery system that needs a gas cap. The filler neck is smaller diameter than the one on my 71 C10. I ordered an "OEM" cap from Rock Auto for the 72 (Stant 10742 deep cam) It fit the filler neck but it wouldn't tighten. It was so loose a good gust would blow it off. So, I ordered an "OEM" for a 71. Stant 10704 shallow cam. it was too large in diameter to even go into the filler neck but it fit on the 71 just right. I looks like I need a cap like the Stant 10742 but not a deep cam type. A shallow cam type might work. Does anyone know what I really need??
Thanks