Before you start doing major surgery, what kind of gear lube are you using in the SM-420? 90 weight straight mineral oil is the recommended lube- - - -if you're using some type of fancy synthetic gear oil, it might be so slippery the synchronizers can't get a grab on those heavy gears and you'll have to shift slowly to prevent grinding. Try flushing the transmission by filling it with Diesel fuel, idling for a few minutes in neutral, then refilling. 90 weight Ford-specification transmission oil for an 8N tractor is the correct lube, or any other straight 90 weight without hypoid additives will work OK.
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!