Bill,
There is no difference in length of the 55-68 OD transmission. What I was refering to is the standard 3 speed non OD trans in 60's (through 67)trucks is about 5" shorter than the OD trans. 68 & up Saginaw non OD 3 speeds also have the long tail housing.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine