The Gear Vendors unit pretty much bolts right on your 4spd. If I remember right, is a .73 O.D. I do know that at 65 I go from about 3200rpm in 4th to about 2400rpm in 4th over. I believe my final drive is equivilant to a 3:08 rear? The extra stuff was that I had to modify my torsion bar crossmember for clearance(probably not a problem for you), shorten the driveshaft, and add a new crossmember for the provided mount on the adapter between the units. I also had to wrestle with their phone techs who gave me a lot of attitude when I told them the 1st coupler they sent didn't fit. After some arguing, I got them to send another. Low and behold, it fit. The wiring was so simple, I could do it. Here are pics.
http://rides.webshots.com/photo/2234118210071636143MfdrhX
