This is for info to use as you can. I recently noticed the plastic piece that the speedometer cable screws onto in the side of my 700r4 was broken off. None of the local parts places had anything but confused looks when I asked about it. The local chevy dealer told me they are out of stock (it's only an 86 IROC for cryin' out loud, not even 20 yrs. old yet). Anyway after a few trips to the junkyards I got one off an Astro van. Wrong number of teeth on the gear of course so I reused my gear. However the plastic transition piece was slightly different. The inside snout was about 1/4" longer (a little time on the bench grinder and some sandpaper cured that problem). The next issue was that the hole (for the gear shaft) was not centered in the snout as my broken one had been. This required the piece to be rotated 180 degrees in the transmission to move the gear/shaft up to the proper contact point with the gear on the output shaft. My gear has less teeth (39) and a smaller OD than the one from the Astro van so when installed with the notches lined up with the keeper the gears didn't mesh. Twisted around 180 degrees (I used one of those square tipped screwdriver tips to insure the gears had a small amount of clearance between them by turning the gear shaft back and forth to get what "felt" like the right amount of clearance) it seemed perfect. I had to grind a small tab down (it had been on the opposite side from the keeper) so the fingers on the keeper would contact the transition piece. Works so far. Beats looking at the SES light when the speedometer doesn't work and is cheaper and easier than trying to convert over to an electronic speedometer. By the way the Astro van piece had some info embossed into it. The little tab (I ground down) had 40 - 45T on it (the IROC one had 34 - 39 on it) and inside the Astro van piece (mine was blank) was:
Black = 40, yellow = 41, green = 42, purple = 43, dark gray = 44 and light blue = 45. Hope this info is helpful to anyone with the same problem.