I happen to have both - a '65 Mustang (heavily modified) and an '08 Vette (bone stock outside of brake pads). I track them both at road courses in Michigan throughout the spring, summer and fall.
The '65 is a lot of fun to open-track - raw, primitive and fast. With the right mods (I'm not going to list them - do a search - that's what it's there for), the '65 surprises plenty of other drivers by showing up in their rear-view mirror and passing. After about 3-years of tinkering, the car's pretty well dialed-in and behaves itself on the track. Given my moderate talents as a driver, it turns in pretty respectable and repeatable lap times. It also attracts a lot of attention at the track - people just think it's cool, and I guess it really is.
Having said all that, short of a ground-up racecar rebuild on that old Mustang, it'll never be as fast as the Vette. On the front straight at Grattan, the Vette is fully 10mph faster into the braking zone; about 142mph compared to 132 for the Mustang (that's a huge difference). That's despite a substantial horsepower advantage for the Mustang (Vette LS3 makes 430hp at the flywheel according to GM - Mustang dynoed at 445hp at the rear wheels; I'll leave the math to you).
The first time I took the Vette to the track, I was honestly surprised by how much faster it was than the Mustang; a little disappointing, actually, when you consider how much time, effort and $ are sunk into the Mustang. All I did for the Corvette is plunk down a few bucks at the Chevy dealer. The Vette's behavior is also way more civilized than the Mustang - heck, I don't even bring tools to the track with the Vette, and I've practically got every tool and supply I own stuffed into the Mustang when I track it.
My recommendation, for what it's worth: Build the Mustang for the road course if that strikes your fancy. Enjoy it. Learn how to drive well. Don't get discouraged when stuff breaks and doesn't work right - just try again. If that doesn't work, try again (and again and again). Keep at it, and your '66 will be a very respectable track car that just oozes 'kewl'. Just don't be surprised if you're not giving the Corvette and Viper guys a run for their money...
- Andy