The SM420 can be converted to open driveline configuration with nothing but a rear bearing retainer and output yoke change- - - - -lots less cost and effort than the ring & pinion swap to the torque tube rear axle. Once that's done, there's an almost unlimited choice of open driveline rear axles to pick from, with a wide variety of gear ratios. If you want to keep the six lug wheels, look for certain years of S-10 4X4 axles, or any of several models of Japanese pickups and Toyota Land Cruisers.
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!