If your 56 truck has the original transmission it would be a Muncie model SM318. I'm not sure if any aftermarket suppliers still make a floor shift conversion specifically for your vehicle. Probably the best you can do is find a "universal" 3 speed floor shift conversion. You will likely have to modify it to clear the seat.
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