The cat40 holder I have has a hole straight through. I just machined the threads off the pull stud on the lathe.

I went ahead and purchased both the gauge and the ram. I used lightweight air tool oil so I could get all the air out easy. I filled the ram and installed the gauge. One thing you must note is that the diameter of the ram is approximately 1.5 inches. This gives a surface area of 1.7671458676442586. So what ever the gauge reads you must multiply by that number to get the corrected PSI. The formula for the corrected PSI is: area = Pi * radius^2. I am gonna search for a 1500 PSI gauge so I can get better resolution.