Just bolt the new engine in and drive. You've got a "3 speed" already- - - -just don't use granny as a traffic gear. The advantage over the the 3 speed is a synchronized 2nd. in the SM 420 that can be engaged without coming to a full stop, something most 3 speeds don't have. The exception is the full-synchro Saginaw 3 speed. If the side cover on the 3 speed you're considering has four bolts it doesn't have a synchronized 1st. gear. Saginaw transmissions have 7-bolt side covers. High gear in the SM-420 and both 3 speeds is still 1:1, so there's no cruise RPM advantage.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!