doing a little more thinking.

original tranny nv3500 1st gear 4.02 x rear 3.23 = 12.9846.
t56 transmission 1st gear 2.66 x rear 4.88 = 12.9808.

70 mph is around 2000 rpm.

that sounds a bit more reasonable. and towing doesn't look like it will be that hard on the transmission with that rear gear ratio either.