I am not saying it is an absolute, but I would start by removing that filter to see if it fixes the problem. Likely it wont, but you have to start somewhere.

Have you tried removing the line from the filter to have free flow off the pump and then try turning it over to see what flow you are getting?

If not, simply run that line to a gas can to see what happens. I had the exact same issue with my 440, I determined that the larger fuel lines affect the psi, combined with the fact that I had a problem with the fuel vapor separator.

If you have free flow from the tank, to the lines to the pump, and free flow from the carb to the lines to that pressure gage to the filter, then the issue is at the filter.