According to everything I have in print the 68 mustang didn't come with 2.73 gears. The standard set for the 8-inch rear was usually 3.00. I was in the same boat last spring trying to figure out what to do with my 73 mustang. I had the stock 3.00 8-inch rear. After realizing how much more performance stuff that was available for the 9-inch I decided to swap out of my 8-inch. I found a 9-inch in a junk yard and rebuilt the third member with 3.70 precision gears and a new limited slip differential. While overhauling the 9-inch I found that the left axle was damaged, to my amazement the 8-inch axles were exactly the same size and both 28 spline. I think the trucks were 31 spline. And with a fresh 9-inch in the back I don't ever plan on having to worry about it going boom.
<a href="http://www.ridgenet.net/~biesiade/Fordrears.html" target="_blank">www.ridgenet.net/~biesiade/Fordrears.html</a>
This address may give you some help with the rear measurements. You can see the 9-inch build-up in my photo album in the photo album section. Good luck, Ford Rules!