Veronica, if you aren't totally paranoid about year correct originality you could put what the 66's got on. My 66 and all the others have a rubber pad that clips onto the inner fender so the positive terminal doesn't touch.
And I am confused about 2 things, what do you mean your battery is in backwards? I thought the Mustangs were supposed to have the terminals towards the back. Also, on the battery cover, is the positive mark on the side that goes by the inner fender, or is it on the side closer to the engine?