Your best source for fuel filters (there are two to a set), will be a local IH dealer (sorry I don't have the part # handy).
The larger of the two goes under the driver's seat on the inside frame rail in the HFCM. You'll need a 36mm socket, and a 6mm hex wrench. Drain the HFCM by removing the drain plug with the 6mm hex key. You should get about a quart of fuel. Drain it into a clear container to see if there is any water or contaminants in the fuel. If it does not want to stop draining, put the plug back in and wait till the tank is half full of fuel or less and make sure you remove the filler cap to help depressurize the fuel system.
You may want to remove the top filter (25mm socket IIRC, or 1/2" socket square drive), before draining the HFCM to allow the fuel from the upper filter to drain out.
The cap on the HFCM is awkward to remove and replace, but it can be done with short strokes of a ratchet. I like to put the rear wheels on a 2x8 to give the nose of the truck just a very slight down attitude, as this will force any remaining fuel in the HFCM to the drain hole, and not back down the filter cavity and onto your arm, as you remove th filter. Place a couple of wadded up paper towels in the frame rail to catch fuel as you remove the filter.
Make sure you replace the o-ring and give it just a bit of oil lube. Seat the filter & make sure the notch engages in the HFCM, and reinstall the cap, taking care not to cross thread. The cap MUST BE FULLY SEATED w/ no gap, but does not need to be torqued hard to seal. Same thing for the drain plug - the seal is the o-ring, and finger tight is fine. I'd highly recommend replacing the drin plug with the IH replacement - its chap, and much easier to use than the Ford plug.