Most of the whine you're hearing is the straight-cut gears of the transmission. The SM-420 has constant-mesh helical gears and synchronizers that aren't nearly as noisy, except in 1st. and reverse gears. The 235 has more torque available, not necessarily more RPM. Both engines max out on HP somewhere in the 3600-3800 RPM range, but most stovebolters who never used these old rigs as daily drivers have the mistaken impression that they're going to start throwing pieces in all directions at any speed above a fast idle. Back when big rigs like yours worked for a living, they ran wide open in whatever gear would pull the load, all day- - -every day. Mt grandfather's drivers pulled trailer loads of beer from Milwaukee and St. Louis to Nashville with 1934 Chevy tractors with 207 engines with mechanical brakes on the tractors, and NO trailer brakes! RPM is what gives you road speed- - - -torque is what accelerates the vehicle after a shift. "There's no free lunch"- - - -either install a more torquey engine so there's more acceleration, or find a higher rear end gear ratio and more speeds in the transmission so the engine won't get lugged down trying to get more speed from an engine that can't pull enough torque to accelerate with the higher speed rear end.
Jerry
