There are any variables to making the frame horn covers work. When dealing with repro stuff.......none of it fits very well. A frame from one place, frame horn covers from another. You get the picture.
Are your covers fiberglass ? ?
Reproduction metal ones are available.
As a side note..........mine are original 32 Ford and I had to heat the rear frame rails and move the rear spreader bar and the gas tank UP about 1/2 of a inch to get a good fit. Original frame, gas tank and all.
There is about 1/4 to 3/8 of a inch between the body and the top of the tank now.