'48 was the first year for the SM420. A '47 has the earlier crash box with the reverse lockout on the shift lever if its still original.
You'll have to change bellhousings to use a later transmission and the clutch linkage may need to be changed too.
I know of no reason to choose the SM420 over the SM465. If you have one I'd run it.
Backing plate width isn't the crucial measure. What is the difference between the widths of the wheel mounting surfaces?
I have a 1-ton with a GM 14-bolt rear with 3.54 gears or something like that. It came to me with a 216 and an SM420. The guy I got it from said the tired old 216 did not have the grunt needed with those gears to haul a load.
Now this was a tired motor, but if that axle comes to you with 4.11 gears or something like that, I'd suggest driving it before deciding to swap the gears, you may decide it needs the gears to do the job.
