While proper venting is a must, tank pressure isn't required for the fuel injection to function. If you are using the original tank your options are few. You must use the cap that is vented both ways and those are kind of hard to come by in the parts stores. And you just can not top off the tank. Your best bet would be to get rid of that tank and put one between the frame rails in the rear. Do some measuring and hit the junkyards. A tank from a Jeep might fit good. I had one from a suburban that fit between the frame rails on my 69 but the Chevy fuel gauge runs on a different ohm range than a Ford so you have to deal with that. Another option is to measure what size you need and have a sheet metal shop brake the metal so it only takes 2 pieces to make a tank. Then anyone who is a decent welder can tig or mig the tank together.