It’s not too small for the paint shop. But it won’t match the quarter. Normally if you want the original color you want something that isn’t exposed to the sun so you get the true factory color. In your case, you want it to match the paint on the exterior of the car, so you actually want something faded the same as the rest of the car. Because the exterior of your car is likely faded to some degree, so even if you had the original paint it wouldn’t necessarily be a perfect match to your car now.
On a ‘73 Dart you should have a rear valance that would be pretty easy to remove and transport. Either that or the quarter extension, that’s also small and pretty easy to remove and reinstall. And both are right next to the part you want to paint, so the match should be pretty good.
