My first question then should be does your mechanical fuel pump have a return line going back to the fuel tank ? It should not, but I have to ask because some models of fuel pumps have them.
I do believe the link you provided for the Actron fuel pressure checker is one that you have to disconnect the line from the carb. and hook straight into the tester, which means you will need to feed gas to keep your truck running.
Let my give you this link to
Quadrajet Fuel Pressure Tech, its an article on what you want to do and what pressures you should be trying to hit.
This is my suggestion for you, get a cheap inline fuel pressure tester, get a rubber fuel line hose and fittings to replace your hard line, do your test and solve any issues you might want to, then replace the rubber hose set up with your hard line and you will be set. Depending on the ends you have on the hard line (male vrs female and such), you should be able to take your hard line with you and match up what fittings you need to use the rubber fuel line.