[AcampoDave]

I think it's important to really be sure what rear gearing you plan to use before investing in a high dollar tranny. Having the right shift points will make for a better driving experience. I would pull the cover off that old differential, inspect its condition and verify its ratio. Most trannys are available with different gear raitio options and what works great with a 4.11 would feel different with a 3.40 for example. What if your differential is in poor shape and in need of rebuilding? Would there perhaps be a more suitable ratio available which would be a better compliment to a given tranny? Gearing is everything to a smaller motor because iideally you can maximize its limited potential by keeping it in the "sweet spot".

Edit: From personal experience, I used to drive around with a very tall rear end and my motor would lug on inclines and I would always be downshifting and climbing hills at a higher than needed rpm. 55 to 60 mph on flat ground was 2000 RPM and my gas milage was not so good. When I had my rear diff rebuilt , I knocked it down a bit changing my 55 to 60 mph speed to around 2300. My little 283 felt like I had given it new life. It was cruising in a more appropriate range and the need for downshifting on mild grades was eliminated. Fuel mileage was improved and I had the benefits of a "peppier" truck.
However, that gear change made the first gear in my old tranny feel very short. I could easily take off in 2nd or go from 1st to 3rd with no issues. I didnt like that, and ultimately bought a new tranny which was geared to play well with my new rear gears. Now, WOW, perfect rythmic shifts and 15 mpg.