Modern Driveline is an excellent resource for five-speed manual transmission conversions for classic Mustangs
If the vintage Mustang you’re driving still has its original transmission, then your freeway performance is not anything like an ’83 or newer Fox, SN-95, or S197—all of which have an overdrive gear ratio for Fourth, Fifth, or Sixth gear, depending on if the car has an automatic or manual.
Whether manual or automatic, pretty much all vintage transmissions have a final drive ratio of 1:1. Even with a relatively tall 3.00:1 rear-axle gear ratio, you’re still spinning about 3,000 rpm—or maybe even more—just to maintain a 65-75-mph speed limit. If you’re trying to keep up with your buddies who are equipped with overdrive, you can forget about it. They’ll just pull away from you like you’re hauling lumber. If your vintage Mustang is an around-town grocery-getter that makes it to an occasional cruise night on the weekends, then a final drive ratio of 1:1 probably doesn’t bother you very much.