I'd say 3/4 ton running gear, just cause you're towing- the Suburban is a heavy mother, then add a trailer and a Blazer, which is heavy as well. 4.10 should be fine, 4.56 since you have OD in that 700r4. If you're towing, driving long distances. Keep that 700r4. They can be built up plenty fine for towing for long highway speeds. 35s and a lower rear gear ratio will even itself out for RPMS down the highway. Somewhere here there's a tire size/gear ratio chart.
The longer wheel base of a suburban will be nice for the trailer. 5th wheel would be sweet for towing, but it being a suburban... nice gooseneck and tandem axles with electric brakes, you'll be just fine. You can upgrade the hell out of a 700r4 to fix all of it's problems. Just don't tow in overdrive, till it's beefed up, and even then on flat highway.
BUT, all depends, how FAR will you be towing. If LONG distances at highway speeds will be the only use for that Suburban, then ya 700r4. Short distances, towing offroad to places to go camping, th400 makes a heck of a lot more sense.
I prefer a manual for towing, but putting a sm465 in for highway use...
All that and NOW you gotta factor in tranny swaps for a diesel. Auto or manual. IF you switch to anything you gotta factor in the RPM range of a diesel. So beefing up that 700r4 makes more sense to me, cause either way you'll be spending money.