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Even a totally stock model A engine is no weakling in first gear or second. I'd suggest doing a compression test. Engine warmed to operating temp, throttle wide open, etc. I seem to recall seeing a chart up on the Ford Garage regarding various different numbers for different heads. Others might chime in here.
Our local club had a car come in with a new engine rebuild that wouldn't run much over 35mph. After a good bit of this n that etc. It was a leaking spark plug. None of the plugs were properly tight. 25 foot pounds on a torque wrench solved that one.
Good luck.