As far as I know, there should be 4 different offerings that should work. the stepside/flare side and Fleetside model of the Ranger should have two different types at the most.
Stepside/ flare side refers to the bedside design. if you have the fender bulges on the outside of the bed and narrow inside, that is the stepside/ flare side bed. it will be 6' long. the filler neck will be shorter, as the gas door is closer to the fuel tank.
Fleet side is a regular pickup bed with the fender bulge inside the bed and the body lines are straight and line up with the cab. there is a 7 foot and 6 foot version, both should have the same length filler neck.
the two variances that you might get per bed type are flex fuel which if you have a 3.0 you probably have the flex fuel, and the non flex fuel if you have the 2.5 four cylinder or the 4.0 six cylinder. that being said, if you do have the 2.5 or 4.0, the flex fuel neck should be fine.. if you have the little green/.yellow/ black corn looking emblem on your tailgate, (and you have the original tailgate) you have flex fuel.
So as long as you have one for a stepside and it at least is flex fuel, you should be ok.
if there are any differences between the three wheelbases (regular cab shortbed, regular cab longbed, and supercab) I am not sure what they would be concerning the fuel neck.
Welcome to TRS,
AJ