The passenger car center section will not fit into the truck housing. It is a completely different configuration. Tri five Chevy passenger cars used several gear ratios. (56-57)PG 3:36, std 3 speed 3:55. Overdrive 4:11. However, The book shows a 3:55 for a 55 PG. Take some measurements, the passenger car housing should fit under your GMC. Remember that it will have a 5 lug bolt pattern so wheels will be required and it will not match the 6 lug front.
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