Look closely at the output end of your 420...is it set-up for open driveshaft, or does the u-joint sit in a cast-iron dish with four ears & bolt-holes, to mate with the torque-ball on the driveshaft?
All '47-'54 Chevy trucks have flat floors (no tranny tunnels), and the tranny-to-bell housing bolt pattern is the same for all trucks from '47 - '66...
You will need to get the floor-shift center floor cover.
Also, the speedo might read incorrectly with the four-speed...the 420 was available in ALL sizes of Chevy trucks, from 1/2 ton thru 2 ton, which covered a range of rear end ratios of 4.1 for the half-ton through 6-something for the 5400 & 6400 series trucks...
Good luck with it !
