On my 86 I also ran into a problem with the filter on the selector valve on the frame rail, it is in an oil filter shaped bowl on the bottom of the valve that screws off. My truck has dual tanks, but If I remember right the single tank trucks still have this "valve" but with less ports, its main function is to serve as a reserve bowl for the high pressure pump as there are no baffle pans in the tanks. NAPA carries this filter under part# FIL3268, it is a round cartridge filter they were the only ones that crossed the OE# of E6TZ-9365-A correctly. Even Motorcraft crosses the number to the inline filter and most dealers are unaware of this filter. I was suffering from fuel starvation problems and my front was groaning so I changed it and still had the issue. I found this filter with help from my dad, he had run into it before. Not saying it is your issue, but it could help as it has probably never been changed due to it's nature. Good Luck!!