Agree, that’s not gonna light up the road. The 38 mph limit is no doubt a feature of probably having the 6.67/1 final drive ratio. Mentioned above is the 4.11/1 custom ring and pinion set that Chuck Mantiglia has available. Unfortunately that set is for the Timken 51524 axle used in tonner and 3/4 ton models. Last I recall Chuck telling me, though, he has NOS 5.14/1 sets on hand. Whether they will work in this early axle he’d have to say. Link below.
Another option discussed for the Bonus Built and later trucks is the substitution of a Dana 70HD axle from a 1970s era motor home, either Ford or Dodge. Same 5 x 8” pattern. These generally have a 4.56/1 final drive ratios. I know of a couple later trucks that were so modified, but again on this early truck the spring perch spacing and spring widths would have to be studied. Stu
http://www.chuckstrucksllc.com/startframe.htm