The 2-speed rear speedometer adaptors for the most part on GM rears are all the same. The ratio multiplier in the rears is a fixed 1.39. If you take the Hi range ratio and multiply it by 1.39 you get the Low range ratio. With that said, there are only one or two adaptors that fit almost all applications, so if you see one in the junk yard grab it even if you don't know what the rear ratio is, chances are it will work.
If you have an Eaton 2-speed rear (these are all electric) the ratio multiplyer is 1.36 and can be found on almost any brand truck...I got mine out of a 60's International.
All GM rears were vacumn shift (at least during the TF years). They have a cable operated vacumn switch that is mounted on the drivers side of the transmission at the base of the shifter. The switch has 4 ports, main vacumn supply, Hi range to the rear, Lo range to the rear and a small port to the speedometer adaptor. The adaptor is mounted right on the back of the speedometer. It has a small vacumn diaphram on it that shifts it from 1:1 ratio (Hi range) to 1:1.39 ratio (Lo range). It's hard to find a good vacumn adaptor, seems like they all have rotten diaphrams and leak.
I have the parts laying around and can take pictures if someone needs to see what the look like.
Hope this helps.
Mike B
