I remember responding to your original post. You had removed the truck trans that should have had the correct side cover. Use the side cover from that trans on the car trans. The side cover is the same on 55 through 59 trucks except for a 55 with a 6 cylinder.
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