It will take a little precision machining to locate the center hole accurately, but it's possible to sandwich a piece of 1/4" steel plate between the SM 420 and the bellhousing that can be mated to a square tubing crossmember bolted to the frame rails. You might need to fabricate a longer than normal pilot bushing to accommodate the rearward movement of the transmission.
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!