Gas tank moving to rear

[bodyman9174]

Ok have done a ton of searching on this. I have a 85 GMC stepside. I mounted the 25 gal blazer tank. That went great tank sits in there just fine. This is were all the post stop. Can not really find any good post on what or were people have mounted the fuel filler neck. I do not want to go though the bed floor. So Went to a junk yard and found a 70 something blazer. I cut the 1/4 off it with the gas cap indent and took the filler neck. Going to weld it into my rear fender. Looks like it will fit no prob. But will I have any problems using that old style gas cap? Looking at the one on my truck looks like there is some vents in the cap. I do have the EEC Carb
Thanks