you have 3 lines from teh fuel tank
1 is the feed line (comes from fuel filter, and fuel pumps)
1 is the return line (even though your pump pushes 80 psi the regualtor only allows 38 psi at the rail so everything over 38 psi, even when the injectors are firing at full WOT) gets returned to the tank.
The 3rd line is the vapor line, vapors from the top of the gas tank are stored in a charcoal canister until you hit WOT, at WOT the computer opens the canister purge valve and the vapors are sucked into the throttle body/intake and the canister is purged....
it is likey that on your truck, like mine, the kid who did your conversion ditched the charcoal canister (holding tank, waste of space IMO, its just there so soccer moms dont smell gas) and ran the single canister purge valve from the old 2.9L on your new 5.0L?
I would have to take a look at it.
Your fuel line setup will be a bit different then mine, my 88 uses two pumps and a filter, in 89 they went to a single in tank pump and where my fuel filter is you have a metal bracket on the frame rail that has 3 metal lines (to bypass the exhaust manifolds without getting as cramped as my 88 style system
i cant wait to see your truck and point out, oh they did this, and oh they did that