When I was building a lot of automatic transmissions I had a piece of pipe bolted to a workbench, and a slip-in bracket that I bolted to the transmission case, with holes and a cross pin that allowed me to position the case at several angles. I also had holding brackets made for several types of 3 speed manual transmissions. It would be fairly simple to fabricate something similar for the SM 420.
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!