I had a similar experience to Chucks with my 1969 428CJ R code. It originally was a raven black exterior with white interior. I change the interior to black. Internally I switched the 3.23 anti-slip to a locker with 4.30 gears (a drivability mistake on my part), but I had essential a SCJ now. My window sticker was consistent with the cars mods. I typically drove the car to shows, but for a MCA National show in Sturbridge MA it was too far to drive especially with those gears. So I open trailered it to the show. At the show they insisted I enter the trailered concourse class and I thought that killed my chances of competing with all the true closed trailer queens. Anyway I received 570 points out of 600 and a silver. I was dinged cumulatively on cleanliness (-6), undercoating (-10), painted vs natural (-5), and a bunch of other minor things. There was no issue with the interior or gear mods as long as they were true with factory availability, consistent with the documentation, or just plain not visible.
So if entering judged competitions is one of your goals, you can make the changes that you want and compete. I used the MCA early model judging rule book for some guidance.