I have one in my 68.
In hindsight I should have just changed the rear gear, but, of all the money and time I spent on my car that's the one single thing that I'm happy with the most. I have a 373 diff. and I can now cruise at highway speed below 2000RPM. If you don't mind spending the money go for it or maybe a six speed.
I got mine from Keisler. I didn't have to fix, modify, change, repair, tweak, massage, alter or persuade anything, unlike almost every other item I've purchased or otherwise obtained for my car.