5/16" factory socks are available & you can put a rubber tipped blowgun nozzle on the sending unit metal inlet & blast it to either blow the sock off or make a split in it & then add a fuel filter as you suggested & on a low rpm app such as yours adding it before the pump "probably" would not cause any vapor lock issues next summer but I think I would add it between the pump & carb which there should be one there anyhow. Taking off the sending unit is such a pain/dangerous I would for sure continue diagnosing & pinpoint the exact problem before I undid it & additionally some pumps wont prime when worn out when cranking because of the slow speed but will do OK when running but the pump is newer & you tried a good used one but the eccentric might be worn excessively. Holler back when you find out, inquiring minds want to know.