Don, I too thought that was as simple as it was. Turns out that there were 2 different diameter drive gears on the tailshafts. Alum glides, original T-10, and some of the early Muncies had a 1.760 diameter steel gear on the tailshaft. With the intro of the TH350 their drive gear diameter went to 1.840 and was made of nylon. They went to the 1.840 in the Muncies too but kept it as a steel pressed on gear for a while but eventually changed it to the nylon gear of the same diameter. This all took place about 1969 or 1970 after the Powerglide was history. Since then all the TH350, Muncie 3 & 4 speeds, and the Saginaw 3 & 4 speeds all used the same drive and driven gears, leaving the aluminum glide and the early Muncie 4 speeds unique with the small 1.760 steel driver that requires the larger diameter driven gear. The more common 17 to 24 tooth smaller driven gear for the 1.840 driver won't work in these.
Nobody was more surprised than me when I discovered there were 2 different speedo gear diameters for 4 speed Munices!!! It must have been a cost cutting / inventory measure to use the same parts as the Powergilde in the early Muncies. Worked real good - look where GM is at today. LOL
