Some of the really low-geared big rigs had governors on them to prevent over-revving in the lower gears, but it was probably sort of redundant once the first few shifts got accomplished. The gas flow through the cylinder head is so convoluted that it's unlikely a stock 235 would hurt itself once it's out of 1st. or 2nd. gear on a SM-420. Maybe downhill with a heavy load, wide open in granny gear might make one over-rev, but it would take a real doofus to try that!
Jerry
