The factory installed styled steel wheel used on the 1968 Mustang were 14x6. It was offered in argent or chrome with trim rings and center caps. Two different center caps were used a plain cap and one with GT lettering. These wheels were of the five lug design and therefore not applicapable to the 6 cylinder cars.
Over the past forty years owners have changed wheels and tire combinations often confusing many people. Additionally, the aftermarket has offered a similar style wheel in various sizes that were not originally offered. Additionally, some of the aftermarket wheels have chrome rims and do not require the trim ring. These are not original.
I hope this helps.