I've got an idea you are sort of backing into the same difficulty many of us have found with this era of vehicle - tank pollution. The line running back there can have rust and crud in it - the end of the suction line in the tank can be fouled - and the tank can be delivering a "fresh supply" of new crud every time you give it a good slosh. You also have to have a vent for the tank too - this is "pre-emission control" stuff. If you run the engine with the gas cap off - its a quick test.The filter you have installed is a means of protecting the engine parts - but more than likely will add to the total restriction. When you open the line - on the fuel pump side of the added filter - does fuel run out (besides the fuel you see in the filter)? On mine - it depends on how much fuel is in the tank ....which indicates that some "head pressure" is required to get fuel to flow.
I tried cleaning out the fuel line - painful feeding of a wire through to agitate the stuff and then blowing through with compressed air and a cleaner (petroleum ether - starter fluid). Crud came out - but it doesn't work much better after the effort than before. Which means....I'll probably have to remove the tank and have it cleaned (along with the equipment inside) - or replace it. I also bought a new stainless fuel line from "Classic Tubes" pre-made to fit the T-bird - such a service may not be available for the Merc wagon.