My '57 with single speed rear 7.2:1 ratio will do 45 mph topped out against the governor, the book says that's at 4000 RPM, I don't think your rear axle would be worst than mine.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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