The correct 4 speed for your truck will have mounting lugs for the brake/clutch/master cylinder bracket. The SM420 bolt pattern is wider than a stock trans for a 40. You may have a 47 up bellhousing on that 228 which does not have the mounts for clutch and brake pedals.
The 3 speed is half the weight of the 4 speed and the 3 speed has syncros in second and third while the stock 1940 4 speed has none. Unless you want that granny first gear the 3 speed may be a better choice.