My '54 GMC 2 Ton has a 5531 backing a Clark 5 speed,also with Overdrive;it seems to run pretty happy with the Clark in 4th(Direct) and the Brownie in Over (0.74:1). With 38" tires and a 6.40 two speed,it gets about 66 mph at 2750 RPM. What tire diameter and axle ratio are you set up with? Here's a link to the number cruncher I like to use to figure this stuff out.
Rocky Road Outfitters.com
Speed


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