1968 340S Coupe 4spd

IMO, since you are replacing rails, the easiest way is to use factory metal. Whether you need to patch portions or replace one entirely, the factory metal will fit better, require less fabrication and after the fact - look better.

I used a set of the AutoRust torsion bar caps once, and they were crude and ugly. A recent article in one of the Mopar mags shows a Challenger getting their rear framerail caps - and they looked crude.

You probably can get some nice rails from crackedback (as noted above), shipped to you for the same or less than the caps.

Grant