That sounds pretty close to what I think my 71 runs at 70 when I run it at 70 which isn't often because there is a big difference in gas consumption between 60 and 70 mph in my truck.
This chart
http://www.csgnetwork.com/multirpmcalc.html can be used to calculate the differences in engine rpm with different gear ratios and tire sizes.
I'm running 4.11 gears now with 245 70 16 tires and plan to go to the 3.56 rear end that I have waiting to go in. With the jump in gas prices it may go in sooner rather than wait until I put the Cad 500 in the truck. You don't really need the 4.11 gears unless you pull loads all the time or haul heavy loads or a camper a lot.
I checked mine and with 4.11 gears 30 inch tall tire and 70 mph I would be turning 3309 rpm with 3.56 gears it would turn 2866 which is a 443 rpm drop at 70 for changing rear ends.