Purchasing tires is a critical issue, especially if you plan to show your car. MCA rules states you cannot have "P" metric radial tires on your 1968 Mustang, in fact you will lose 5 points automatically if you have "P" radials. Radials are legal for 1968 cars, as long as they don't have the "P". Stay away from bias tires if you plan to drive your car. If your car is strictly a show car, E70-14 wide ovals would be correct.
I run BF Goodrich Radial T/A tires with a size of 205/70R14 on my CS. This size of tire is close to the original size, yet wider, and the tire fits well in the wheel well. I have been deducted for my tires when entering MCA shows, though.
Hope this helps,
Russ