MCA generally will allow over-the-counter accessories such as dress up kits without having documentation that a particular car came with it new. At least in the 65-70 Mustang concours classes.
The only caveat with MCA is that excessive items could result in a deduction. Normally 1, maybe 2 per major section are allowed, but I don't think that is written in the rules. Basically, it is frowned upon (potential deductions) from adding too many of these items as the car becomes something that simply wasn't commonly seen when they were new.
Otherwise, I agree with Bob that the whole idea of concours is to restore a car back to what it looked like when delivered to original owner when it was new. If you're not sure if the car came with the Cobra covers and have no documentation, then the default would be to go with stock chrome covers.
On the re-chrome, if you have a super nice set of original steel covers, ask the chrome shop if they can do flash chrome as they were done originally. This may or may not reduce the cost slightly. These covers had a lot of imperfections in the surface from the forming process/mold and if they are chromed perfectly, they really don't reflect what they originally looked like.
