All great ideas. Another is to pickup some ladies finger nail sanding sticks. They are usually very inexpensive and most drug store or even the market carries them. As pointed out the very fine ones if you're doing the side walls. Another thought, After sanding the tread mask it off with tape and lightly spray flat clear on the side walls.
The wheelbase doesn't look that far off. Is the NASCAR chassis to wide to fit under the running boards in the middle of the body ? Just from the picture it looks like you could shorten the race car floor boards behind the gas tank and remove the ducting for the front radiator and it might just fit. The chassis may still be just a little to wide in which case you could narrow the race chassis just in front of the rear tires and where it angles out behind the front tires. Just an idea if it fits with what you're thinking.
On the smaller parking lamp lenses the white paint is probably the easiest. On the head lights a couple of easy things to try, paint the lens with a semi flat clear or even a flat clear. This would give it a less shinny look compared to the chrome trim ring. Beyond that you could drill it out and use a spare headlight lens.
This is one of my personal 1:1 favorites and the next one to build. I like your color choice and your paint finish looks good as is. What colors are you using for the interior ? The engine looks fairly accurate but I question the general shape of the kit's valve covers. The creases on the top of the valve covers I remember were shaped a little differently, but they were the standard 389 cu. in. and that could be the difference.
Another Meguiar's product to try is called Plastx clear plastic cleaner & polish. It is designed for taking scratches out of plastic instrument covers and tail lights. I'm sure it would work on head lights also. I used it on an old Monogram '64 GTO molded in red. I trimmed the mold lines as much as I could with a #11 blade followed by fine sanding sticks and a 3200 sanding pad. This leaves you with just a little polishing and maybe some waxing.
It maybe that Testors is doing colors that can be done in just one shot. Candy colors are usually dependent on a base coat of a high metallic silver or gold, and a transparent color coat of what ever shade you're going for.
Everyone needs a place to go when they feel the need to get away. It will be different for everyone. For myself its building models. I'm fortunate that I have late model automobiles needing little more than normal maintains. It hasn't always been that way as at times in the past I have spent far to much time in the garage.