Want to shed a little more light on your 2 PSI readings at the current location/conditions. First to get a system pressure you need to deadhead the output of the pump (if any of the 4 needle/seats on dual carbs is open the reading is not correct). The pressure output reading of the pump must be checked with all seats closed, but engine must be running for mechanical pump to work. The carbs fuel bowls hold enough fuel to idle long enough to get a pressure check so not that tough to do.
While being careful of spilled fuel ... yada yada. Place short hose with gauge on the output of mechanical pump and start the engine running on fuel in the carb bowls. The pump has internals to keep it from blowing up, it will only deliver so much pressure and that is what you are checking.
Flow or volume potential is another issue. A pump can deliver the correct pressure but not deliver the proper volume. The method to check volume is to run pump for a fixed time and measure the output volume in given time frame. Look in an older Motor Manual to get spec's on say a 1966 with your cubic inch engine and set up. I know you are not working on a chebbie but easy to find for say Z/28 with multiple carbs and Cubes are close.
Now as far as gauge accuracy and temp I think a quality gauge will get you within <5%+/- of sweep =
http://www.autometer.com/download_instruction/425f.pdf after equalized to your area and ave temp.