For 1966 Mustangs, Ford added a rubber pad on the right hand fender apron near the battery as an insulator, the battery hot cable can touch the fender apron and raise havoc. If you replace the right front fender apron, you may get one without the four holes for the pad. Here's a sketch I created in 2003 to position and drill the holes for the pad on a 66 Mustang that had severe battery corrosion on the fender apron. I used two Oct 65 San Jose Mustangs to get the dimensions.
Note: The Ford Part Number for the pad is a 65 number - C5ZF-10K653-A. However, it first appears in the 66 Mustang Electrical Assembly Manual (AM0013 page 15). The pad may have a TSB adding it to the fender apron in late 65. As a service number, its base number, 10K653, does not appear in the 60-68 or 75 editions of Ford Car Parts (aka MPC) as a service part, so it was most likely a "dealer, fabricate something that works" item.
The pad and retainers are available at many Mustang parts sellers.
Jim