The 216 might be a little short on torque to handle a 3.55 gear ratio. Another option might be to switch to an open driveline setup by swapping the rear bearing retainer and output flange of the SM 420 and going to the 3.90 rear end used in the later model stovebolt pickups. With an open driveline, you would also have several choices of newer 6 bolt lug pattern rear ends with a variety of ratios to choose from, while still being able to run original looking 6 bolt wheels.
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!