I would look for a crushed metal return line under the car or a kinked return hose but you may be doing all of that for nothing if the gauge is bad...
I would just look into renting one of the fuel pressure testers, no point replacing it if its accurate. You should be able to rent one from local auto part stores like autozone or orileys. If you cant get one, I would look into getting one from harbor freight or amazon. If you already have a gauge on the rail then you already have the adapter you need to connect a pressure test kit.
Make sure to check the whole return path. The most common place for kinks is where the return and supply line loop back up near the front passenger side of the engine. Its possible someone kinked the return line back at the tank, or its pinched due to incorrect routing. Its possible the metal lines that run down the front of the passenger side of the motor are damaged but based on the pics, it looks like there are custom lines. Like you said, you never know what someone else did. If you end up dropping the tank to replace the pump, you should be able to get away with a stock 88/95lph pump. Slightly bigger isnt going to hurt but you really dont want to go significantly bigger than you need with a fuel pump. It minimizes heat transfer into the fuel. A 110 lph would be a good choice or even a 150 but you really dont need anything crazy based on your current setup. If you plan on adding a good amount of power to it soon, a 190 would probably be more than enough unless you plan on making a lot of power (450+)
You could go back to the stock lines for ease. They are good up to about 500hp. There are adapters to connect the stock lines and rails using an fittings if you're missing the connections. I have the metal lines that run from the rails down the front of the motor if you're interested in them. If not, new aluminim lines would be the easiest and cheapest way to do it but it's still pretty expensive to do.
