Are you sure that isn't a 5.13 ratio rear? That's what it should be if it's from a 1 ton. Go advertise on the 67-72 Chevy board (
http://67-72chevytrucks.com/vboard/index.php ) that you have a 5.13 that you'll trade for a 4.10. You might get a bite. Also look on Pirate 4x4. Turns out the 4x4 guys really want these 5.13 ratio gears. They are more rare than the 4.10's. The axle is highly desirable, as it is a super tough "bullet proof" axle, with decent ground clearance.
I've heard rumors of a 3.90 that only fits in a 67-72 carrier, but I've never seen it, much less seen one for sale. I'm keeping my eyes open though. The 4.56 is a compromise from the 5.13, but won't give you that much. The 4.10 with stock rims won't get you that much speed. Your still not looking at going out on interstate.
The only way FOR CERTAIN that I know to determine the gears, is the lift a wheel and spin and count. No... that's not true. You could take out the carrier and count the gears... but that is much more labor intensive.
I've got a 4.10 with correct 17 spline Detroit Locker in transit to my house today! This will go on my 58 1 ton. I have the same issue, wanting to drive faster than 40 mph. The gear should help. The Spicer Auxiliary (with 0.8 OD) I've pulled off a doner should help even more. With the auxiliary I've got a 2.3 Lo Lo gear, that will give me stump pulling low that I'd otherwise lose when replacing the 5.13 with the 4.10. Then with my main in 1:1, auxiliary in 0.8, 4:10 gears, and 19.5" tires, I should be able to drive 60 mph at 2000 rpm. I DON'T plan on driving that fast while carrying 2 tons of stuff. Brakes are still stock.
