Well, I would personally balance it by picking cars by category and coming up with a handful per decade. Well, in this case I would apply that with every decade from the 1950s to the 2010s. There are probably not too many people clamoring for cars made in the 40s or prior, so no need to go as crazy. I would just pick some of the most significant cars from those early decades like the Ford Model T, Bugatti Type 57 SC Atlantic, or Mercedes-Benz SSK and move on.
I don't know what your point is, but if the point was that it can be a big hassle to truly balance the list by decade, then alright.
That's true, though I think the big mistake on that was doing that when they had to start over again with a smaller car list. Had they waited longer to try going for a race car for each brand, like in this upcoming game, it probably wouldn't look as bad. Especially with how popular GT3 is with sim racing.