Chevy trucks used the same bell housing mounting pattern for all 3 speed and 4 speed manual transmissions from 48 up. This includes V-8's and 6 cylinder applications. Cars changed in 55 to the same mounting pattern. If the supplier doesn't know this fact, they are probably not the right source to purchase an adapter from. You should also get some guidance on using supposed similar bolt pattern applications on Ford & Chevy transmissions. Someone here should have that information.
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