I think the simplest way to check the actual oil pressure would be to drill and tap the filter cover and install a manual gauge, kind of like this:
This particular image shows a 617 but it should be no different on a 603 with a gauge fitting. Ideally you could find a oil filter top at the junk yard, but they are not nearly as plentiful as the 617 ones. When you're done testing just screw in a plug. There are several threads on the subject and all recommend drilling from the bottom to avoid the ribs.