i recently bought an 85 f-250 turboed with a new c-6 tranny. I want it to stretch its legs on the highway and am contemplating a gear vendor or upgrading to e40d if that is a viable option.

 

Since you have a turbocharger, I would not recommend an E4OD tranny unless it is an aftermarket or has the minimum internal upgrades.

When I bought my truck, it had a C6 tranny, and 4.10 gears, no good

First thing to go were the gears, but I have a 2wd truck, and swapping to 3.08 gears was a natural fit. If you have 4wd, or intend to ever swap to 4wd, 3.08s will not work (ford HD front axles only go to 3.54).

The E4OD swap probably ended up costing me a round 3 grand (CA$), and I am happy with the results: 1650 RPM @ 70 MPH. The E4OD has a 29% overdrive ratio, and a wider 1st and 2nd gear ratio.

The gear swap cost me $400 CAD (back then it would have been $250 USD) delivered to my door, but in both cases, I did all the work myself. With 3.08s, and the C6, I could manage 2400 RPM @ 70 MPH. I ran the truck for over a year on the C6 and 3.08 gears, and never once regretted the swap, but since most of my driving is above 70 MPH, I still wanted lower RPMs.

If you need to tow more than 5000lbs consistently, I would not recommend going as far as 3.08s, but for running empty, I noticed no difference on 0-60 run time compared to the 4.10 gears.

I have never used a gear splitter/overdrive, so some one else will have to weigh in on that one, but they are not cheap.

 

you could make the swap , trans needs to be an 89-91 model 92 had no output shaft gear for the speed sensor and speedo cable drive and of course the processor and wiring loom