The clamp for a 65 V8 according to the 65 Electrical assembly manual is p/n 357949-S. It is shown in the Ford Standard and Utility Parts Catalog, January 1965, on page 37. The dimensions are: 21/32W, 1 3/64L, 27/64 Loop, Beveled end, w/mtg hole, rubber coated. Its only Mustang application is 1965. AMK does not carry it.
The mounting screw is: 42119-S8 hex, tapping 8-18X3/8L, flange 11/32, 1/4hex drive. It is the same one used to hold the heater plenum to the heater.
Jim