55-65 are non syncro, 66-69 are Saginaw full syncro units.
Just get the most complete one you can. Sometimes you can get lucky and everything works. I have repaired many solenoids and govenors and aftermarket electrical components are available. Lots of info on this site about this subject.
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