So I finally got my new gascap, for a 66 GT, I thought it was going to be easy to put on, so I try to unscrew the other one off, then it wouldn't come off, so I looked from inside the trunk, there is this nut holding it on which is unreachable. Has anyone changed a gas cap, if so can you please help me.

Then i figured if I got the nut off, I'd have to put the new nut on, which means I'd have to screw it on somehow in the place I can't reach, is there something that you take off that makes this easy, or a special tool to do it with.

Thanks

 

Looks like a mod to make up for an enlarged screw hole on one of them. If that's the case, you will have to improvise to keep the nut from turning. Try hemostats or even a small screwdriver...