I did a lot of towing with a 59 Burb with a completely stock 1970 307 2-barrel engine and a SM-420, including a 22-foot Norris camper that weighed in at close to twice what a modern RV of similar size would be. That thing was a real lead sled! No problems at all! However, I was using a Dana type 60 3/4 ton rear end with a 3.73 ratio and a limited slip carrier. Your 3.90 will be a pretty good towing gear, but I question its strength with the engine/trans setup you have. I'd recommend going to something like a 9" Ford rear axle, or something similar from a GM pickup a little newer than the original rear axle. A ratio somewhere in the 3.7:1 range will give you good road speed, and the granny gear of the SM-420 will handle getting moving on even a steep upgrade.
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!