The side cover is the same except for the 1-R shift fork. The slider gear is wider on the later trans hence the fork is wider.
There are two types of side covers: The ball type and the scissor type. Internal parts don't interchange but if both of the ones you have are the same, you can switch the parts. The 55-2 V-8 side cover is different than the 6 cylinder. The early truck side cover shafts are clocked with the 2-3 shift lever pointing up and the 1-R shift lever down. On V-8 trucks and 56 up 6 cyl both levers are down.
Fred

Last edited by rfs56trk; 12/22/2017 4:38 PM.

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