Honestly, except for the two Muscle Car races early on, I haven't seen where the muscle cars are of any real use except to collect. I wish it were different, but it's not.
How is it still a Shelby? Same HP? Same suspension? Same weight? Same anything besides the artwork that makes it look like a Shelby in replays? (I bet it has ABS, traction control, and a computer chip in it too huh?) LOL
I'll be the last person to tell someone how to play their own game, and I myself enjoy building the occasional hotrod. I just don't see the point of turning a classic car like that into something totally different. If you want something that drives like what you ended up with, why not just get that car in the first place? There are probably hundreds of cars that have the same specs as your "tuned" Shelby.
To me, the whole point of having so many cars in the game is to... have different cars in the game. The power and handling (for better or worse) is what makes them individual and gives them a personality. By throwing the whole tuning shop at them, they lose that personality and you're left with just another boring car that drives exactly like any other tuned car. And you also take a little away from yourself, by not learning how to drive and respect these classic cars for what they were.
But like I said originally... whatever floats your boat!