You've either got a locked up 3-4 shift rail, or the 3-4 synchronizer is jammed in the neutral position. Take the top cover off the transmission and see if you can move the 3-4 synchro with a pry bar. It's the one closest to the front. It looks like you've got a transmission overhaul coming up in the near future. Fortunately, most SM-420 parts are still available. Not cheap, but still out there in pretty good quantity.
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!