I switched mine out for a 3.64 ratio. Could even went up to a 3.73. I still don't use 5th unless I'm on freeway doing 75mph. The best thing is to buy another unit from a good company that does the rebuild. I happen to find a good deal and had it sent to Jay at Vintage and had them install it.
Here is a great web site. Use it to find your ratio. Alittle research on your speed and tach and you will have the perfect ratio.
www.diffsonline.com/techinfo/index.shtml
I hated that my car would only spin one tire unless I unloaded it. Now you better hold on. Yee ha
Good luck
tom