My 81 Jimmy has the 6.2, and 4.56's and had a SM 465. Cruising at 65 mph it got a steady 16.5 mpg...no more and no less. This was on 40's, and the rpm's at about 2500 or so. On a 37" radial it might have seen 18...
It now has an NV4500, and although I haven't driven it in a while and never got a chance to figure highway mileage with it I'm pretty sure it'd pull down 21-22 mpg on the highway now.
My 83 C1500 cruises at 1800 rpm and regularly gets 25-26 mpg on the highway. The TH400 is a great tranny for being strong, but it also eats a bunch of power and does not have a lock-up convertor so it is also a fuel mileage killer.
Rene