I'd hate to presume you'd take suggestions from a podunk car guy like myself but if you don't mind, I'll offer some anyway.
There are a number of things that I'd like to see retained from the original interior:
I feel that keeping the stock steering wheel would help to maintain the 60s vibe. I could see incorporating a modern logo on the center cap, paddle shifters, or perhaps a modernized shift indicator added to it but I'd hate to see the entire wheel swapped out.
72Valient4Door recently posted a picture of a dash pad he'd had an upholstery shop make for him. Although he got some negative responses when asking other's opinions (including yourself), I thought it probably looks much better when the view isn't such a close up. Then the pattern on it might not seem so pronounced. - Now, I'm not suggesting an ostrich skin pattern but I think a stitched seam line on yours would subtly add a modern detail and still retain the original contours. If the stitching theme was carried over to the rest of the upholstery it would tie together nicely.
The dashes in 1968 incorporated padding on the bottom which I think would also add a modernized touch if they had stitching that matched the rest of the interior.
I know that seating from a new car would be a step up in comfort but it might be neater to see an original pair of buckets modified to wrap around you more and given a more attractive sewn pattern. - maybe some emblems sewn in to an added set of headrests.
The 1967 dash cluster isn't ugly but it could use an upgrade. The textures on the plastic could use some sprucing up. It's not a major change but I added a carbon fiber overlay on the top half of mine and a new silver overlay on the bottom. I suppose the colors could easily be changed to something else. If I'd invested in new gauges I think my dash would have had a more modern look.
I don't think Dakota Digital makes new gauge panels for them yet but it'd be great if they did. We bought a B-body one for our '68 Charger and I think they're great. Perhaps you could still manage to install newer gauges in the factory cluster insert. We didn't want to lose the look of the original cluster in our Charger, but we wanted better than original.
With your car being set up to handle like a track car you'd said you'd debated retaining the rear seat. The lack of necessity having one makes it easy to decide to get rid of it but then you'd be eliminating an important component that has the car appearing it came off the showroom floor.
If you go too far with the door panels you may lose some of the feel of cars this vintage. I don't know how much trouble it would be to add in modern armrests and storage pockets but it might work. - Heck, I don't think I'd rule out adding power window switches and ditching the cranks.
