Taking all of the above into consideration, my suggestion to use the original chassis and a Cadillac/TH400 powertrain sounds better to me all the time. I towed an 18 foot 3 axle trailer loaded with all our worldly possessions from California to Tennessee back in 1980 with a 72 Cadillac Fleetwood Brougham, with a 472/400 powertrain. My wife drove our 59 Suburban with a 307/SM420 and a Dana type 60 rear axle, and towed a Datsun station wagon. Gross weight of the car/trailer combo was 13,700 pounds. The rig never missed a beat, but it only got 5 MPG for a 3K+ mile trip! The Burb got 10 MPG. Drive 100 miles and fill up with gas- - - - -"Rinse and repeat!"
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!