I would try the sending unit - don't know without looking it up if there's a way to test instead of just replacing.

Another possibility is to buy a cheap mechanical gage and install it temporarily - again, I don't know without looking it up where the sender is located and how easy this would be to do.

My only other experience with this was with a Ford 351 years ago where the pickup screen got glazed over and blocked, but the pesssure was "0" always with that problem.