The original rear end is a 4:11, at 2750 or there about with 6.50-16" tires gets you to about 55mph. The granny gear in the 4 spd tranny is mostly for "pulling stumps" and is not really need for regular driving.
If you are wanting to cruise at 70 mph you might need to upgrade the brakes for disc on the front anyway. These old trucks really were made for 70 mph. They lack the handling and braking for that sort of thing without some upgrades.
To go with an open driveshaft rear end would also mean changing out the transmission for something newer 55-up. I believe you can change out the pumpkin with a later 55-59 truck rearend (3.90) which they do make a 3.08 conversion for.
John
