Here's what I found...
It's hard to match up model numbers with casting/part numbers, but I think I figured it out...
Case Casting C-91171 is a 1956 only Chevrolet Series 5, 6, 7 & 8 New Process 5-speed (direct drive). That would make it the NP-540 as it holds 10 pints of oil. The 1957 Series 5, 6, 7 & 8 NP 5-speed has case casting number 92116.
I ruled out the NP-435 as it only hold 7 pints per my MOTORS Manual and I can't find any Chevrolet NP truck transmissions that hold 7-pints...
I also ruled out the 465, as that number SM-465 is a corporate GM 4-speed transmission made in the Muncie Plant from 1968-up.
The cover number is a good number used from 1956 at least thru 1967 (exc. TILT).
You'll get one additional gear for better puling power, but you won't gain any speed going from the SM-420 to the NP-540.
Mike B
