According to the 1959 truck resto pack your truck rear end had a Regular Production Option RPO 3.70 and 3.38
You could find a 3.38 ring & pinion 3rd member and install in the axle housing and you could drive at 70-80 mph all day long.
The Resto pack does not seem to show the RPO number for the 3.38 RPO, Teeth 13-44
1959 Resto pack PDF page 41.
1959 CHEVROLET TRUCK OPERATOR'S MANUAL 1/2 Ton Rear axle ratio spec. (bottom of page)
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