Are both internal shift levers the same for the 1970 GS Muncie 4-speed transmissions? I'm referring to the levers within the side cover with 3/8-16 internal threads to which the flat external levers attach. In 1969 they were the same (part number 3950308), but in 1970 the TCS switch appears which was operated by the 3rd/4th shift lever. Auto Gear Equipment's excellent catalog of Muncie 4-speed parts shows part number 3950472 for this 3rd/4th shift lever but they don't call out the 1st/2nd shift lever (for the short shift tower shift cover w/TCS).
If you don't have your original, or a year-correct, Buick manual transmission for your 1969 or later GS, then you may run into problems when attaching the back drive hardware to the transmission. If you're doing a 4-speed conversion you'll almost certainly run into this problem. The transmission gear shifter shafts changed in 1969 to 3/8-16 internal threads to attach the back drive hardware, and finding these correct shifter shafts is tricky. Auto Gear Equipment reproduces the earlier shifter shafts that won't work, as they accommodate 5/16-24 studs. Also, according to Auto Gear Equipment, the 3/8-16 internal thread shifter shafts come in two lengths, one for short shift towers and one for tall shift towers. I believe that our 1969-73 manual transmission Buicks all use the short shift tower side covers. Why do we care about this? Recall that the back drive (1) turns on the reverse lights when shifting into reverse, (2) turns off the reverse lights with shifting out of reverse, and (3) locks the steering column and allows the ignition key to be removed once the transmission is placed in reverse. Looking at a Buick parts book, dated May 1972, section 4.309, shows part number 3952651 for the 1970-72 C.T. 4-speed transmission shifter shaft. Looking at a later Buick parts book, dated April 1983, again section 4.309, shows a different part number, 3952649, for all 4-speeds from 1969 through 1973. This same parts book shows 3952649 as the 2nd/3rd shift lever for the 3-speed manual transmission for 1970-74. As I mentioned in my initial post, Auto Gear Equipment's catalog shows part number 3950472 for the 3rd/4th shift lever. So at least part numbers 3952651, 3952649, 3950472 and 3950308 (without the TCS notch) work for these Buick applications. I pulled 3952649 from a 1972 Skylark 3-speed manual transmission, and purchased 3950472 online. Both of these levers have a "notch" machined in their shafts for the TCS switch. Both looked identical, except for the placement of the notch on their shafts. Both installed nicely in my Muncie M-20 4-speed short shift tower shift cover with the TCS switch(casting 3952648), meaning their height was correct for the short shift towers with the flat external levers just clearing the side cover short towers without rubbing on the towers themselves. Please refer to attached photos. I've yet to install the transmission in the car and drive it, but I have bench tested it successfully by installing the shifter and linkage and running the transmission through all the gears, including reverse. Lastly, these shafts are hardened steel, so only consider drilling and retapping them if you have access to a machine shop.
