If you want a synchronized 1st. gear, retro-fit a Saginaw 3 speed from a 66-up truck. It will also involve some tinkering with shift linkage if you want to retain the column shift and probably a change of driveshaft length/front yoke. The way to identify a Saginaw easily is the fact that it has 7 bolts, not 4, attaching the side cover to the transmission. The input shaft spline and bellhousing bolt pattern are a direct swap with the Muncie 3 speed.
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!