I used an Auto Rust Technicians cap for the drivers side front frame rail on my Cuda. The fit was fine, and it has held up well for me - I put it on in the late 90s, and it hasn't given me any problems since.

However, I could have made my own frame rail cap for about 10 percent of what they charged me. Their caps are just 3 pieces of steel welded together so that they fit over your existing frame. There's nothing special about them. You could make your own for 5 - 10 bucks and a little time.

Visually, the caps are ugly. That doesn't bother me, though, since it's under the car and not easily noticeable, and my car is a fifty footer anyway. But if you have a nice car, you might think twice about a cap, since repop frame rails are available now. In the late 90s, the only thing available was used frame rails, so the cap was a good choice then for me. But if I had to do it again, I would get a new frame rail and do it right.