"Go big or go home!" I'm planning a Cadillac 472/Turbo 400 powertrain for my 59 Suburban. The Caddy engines are thinwall castings, with only slightly more weight than a small block Chevy, and displacements from 425 to 500 depending on the year. They're also fairly available in salvage yards and not too expensive. I'd recommend a 3.08-3.55 gear for good performance and decent cruise/top end performance.
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!