I'd agree with that. Lets face it, if the Cannonball was being run today, all the racers would be the most boring, unassuming cars you could imagine. As modelers we're allowed the freedom of our imagination and given a little "creative license" when it comes to picking our racer.
Matt, I would not change your entry. Like my Pacer, your Caprice would stick out on the modern highway, but no one would imagine they were capable of 140 mph. The same goes for most of the racers.
The rules we're using this year are as follows. If we try to over-think this too much, we'll suck the fun right out of it.
1. Take a car that is not normally considered fast. Make it fast without altering its appearance that might let on that it's fast.
2. If the car IS normally thought of as fast, it should be in such poor condition/appearance, that no one would give it a second glance.
That's my $.02.