I figure, 5.14,40" tires, direct drive transmission, gets you 70 at 3,000 rpm
calculate here http://www.onlineconversion.com/bigger_tires.htm

Even with an OD gear 5.14 is way to slow. Just because it's governed at 2,800 rpm doesn't mean it's a good idea to cruise at that speed, you'll just make a whole lot of noise and waste fuel. For a 6V53 non turbo 1,800-2,000 should be a suitable cruising rpm.

In the F350 I used a 12,000 lb Rockwell from a small Freightliner. http://rides.webshots.com/photo/2287711350080251109kBIUJt
I also used the Freightliner spring suspension, which might be a good idea for you too, as you want to move spring hangers any how. Or better yet find the rear axle and suspension from an ambulance, RV, or moving truck with air ride, and likely the same 12,000 Rockwell I had, which can get as fast as 3.31 gears, I had 3.73, and 3.54 are common.
Here's the front and rear axles I used in the truck http://rides.webshots.com/photo/2203512130080251109lwAcFd
And the whole album showing the swaps http://rides.webshots.com/album/558384968mIheUE

All the NPR axles I looked at were 5.something or high 4 at the best, check them by turning pinion and hubs and counting, you don't need to disassemble them.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-