Depending on your hiway rpm, it's a heckuva lot cheaper to swap rearends.
Or even to mod the engine for more bottom-end torque.
Fixing the slipping-out business is easy and cheap. I have been running the same gearset for a long long time.
Without knowing anything about your combo, it's hard to make any recommendations, but;
I found that in the early sixties, there was a gearset that had a 3.09 low gear in it, and I refer to it as the Commando box. The other three ratios are very similar to the later 2.66low box.
The difference is 3.09/2.66= plus 16%. This allows you to take out the same 16% rear gear for very little performance loss.
For example; just spitballing here;
Say you had 3.91s now,with a 2.66 box
(2.66-1.92-1.40-1.00) Your roadgears are
10.40-7.51-5.47-3.91..
Swapping to the Commando box
(3.09-1.92-1.40-1.00) and 3.91 less 16% =3.37, which rounds down to 3.23s,or up to 3.55s. Lets go 3.55s.
Your road gears are ;
10.97-6.82-4.97-3.55
(With 3.23s they would be
9.98-6.20-4.52-3.23)
Your hiway rpm drops to 3.55/3.91=.908, so if you were cruising 65=3233, then, now you will be cruising at;
3233 x.903=2919, (2670 with 3.23s)
Your starter gear is about the same. but
your second gear is a lil slower to the shift. However, there is a good chance it is a better gear to hit 65mph with.
With the standard box, 65 in 2nd is 6210
With the Commando box, 65=5640, which;
Depending on your cam, could be quicker to the stripe.
This is what I did, and have never been sorry.
There is one hick up, maybe two.
1) the Commando comes with the old-style Ball&Trunnion output on it, so you have to rob parts from your standard box,to make it work, and
2) there are zero gears that can be swapped between these two boxes. Well except for reverse,lol.
I called up Passon, and he had what I needed, and I right away also ordered a brand new HD-aluminum case and aluminum cover from him. I thought the prices were reasonable.
I tried the 3.23s with that 3.09 Commando box, and I really liked it. But I have an HO 360 with BB type low-rpm torque.. For anything less, I think 3.09x3.23=9.98 will still be Ok, but if your coming from 3.91s, you will notice the loss off the line.
You can occasionally, score a Commando here on FABO, very well priced. Downright cheap compared to the options.
In the end I installed a GVod for 65=2240rpm,lol. But that wasn't cheap back then and even less so today.
Please recall that the above is just a math exercise because I have no idea what rear gears you have. But the 3.55/3.23 part of the exercise holds true regardless, and all rpms reflect the use of an 83" tire roll-out, which is about 26.4 diameter, a number I picked out of a hat, but is close to a 225/70-14.
Jus trying to help
