You have to remember that auto trans had not been around that long in the 60's. Remember the dodge trans in the 50's? I think it was called a dynaflow. It was a mix of automatic and three speed. Many people just did not want one regardless of income level. Why would a farmer want an automatic? Most farmers I knew had gobs of money but they really weren't into fixing stuff unless it really needed to be done. Most of the Grain hauling trucks were 6cyl w/manual trans that way you could stop the damn thing easier by leaving it in gear and shutting the engine off. They did not seem to want to fix brakes.
Anyone remember that Cadillac offered a 3 spd trans in a coupe deville up until something like 72. That was for the farmers. also the amish farmers would buy a black one with no options