The original 3 speed transmission is about 22" from the bellhousing mounting surface to the end of the tail shaft.
The SM420 is about 14". As mentioned previously the front yoke and u-joints are completely different.
A modified or replacement driveshaft will be required.
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