Cat 3208 435 hp with Air Seps
Sounds good. I would suggest skipping the adapter fabrication, and get the crankcase pressure at the dipstick tube using a short piece of tubing to connect the manometer. That's were all dealer and mechanics troubleshoot from...system vacuum will be vacuum, pressure pressure.
I only mention the manometer since the difference will be so slight, that a PSI gauge..may not cover the accuracy of say 2 inches of water.
Your absolutely right about the keystone rings...and if no other mechanical issues are present, and you run low power all the time. Switching to a slightly smaller filter element may improve things.