My understanding is that they changed the fuel tank/ filler from '76 to '77 and again in '78 so '77 was an odd ball year.
I ran into the same problem you had when I replaced my fuel tank, luckily we were able to extend my filler tube and it worked fine. When I replaced my passengers rear corner panel, I ran into another problem. My stock filler protector was round and in a slightly different position than what was on the new panel with the square filler protector. I have it a bit rigged right now but it works, I just can't put the skid plate/ cover/ protector on right now. I still need to go back and figure it out...
old: (I no longer have that bumper, FYI)