You can use your original side cover on the later transmission to match your existing linkage. However, you need to replace the 1-R shift fork with the one from the 55 trans as the gear is wider.
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