new fuel pressure guage reading low...?

[DLangford]

My 2001 high output CTD is running well right now. A few years ago, I experienced the lift pump starting to fail, and got it in under the warranty with one month to spare. The dealer installed the in tank lift pump fix that Dodge was doing at that time.
This past weekend, I had my neighbor (who is a pro mechanic) install a trio of guages; boost, exhaust temp and fuel pressure. All are Westtach from Geno's Garage.
The boost and exhaust temp work fine. The fuel pressure guage (electric), with the recommended isolater was carefully installed according to directions. My problem is that the readings are low- about 4 or 5 psi if the isolator installed at the injector pump, and 5 to 6 psi if installed at the fuel filter. When he saw this, my neighbor then bled the isolator and guage to make sure there was no air in the line. That did not "help".
He then went on-line, and found a mechanic's "bulletin" from his normal site he uses everyday, that implied that fuel tank mounted pumps in the CTD's will give you falsely low fuel pressure guage readings up near the engine.
Has anyone had a similar experience? Advice?
I am planning on swapping the fuel filter just to make sure that is not part of the problem, but I am sort of nervous with such low readings, even though the truck is running well now, as I said.