If you are unaware, they have two gearboxes, one behind the other. Basically you go through the gears in the "main" then shift the auxiliary up a gear, then start all over in the main. "Sort of." Because there are certain combinations of the two gearboxes that cause overlap, that is nearly equal ratios with some other combination of the two, some combos are skipped. 3x4, 4x5 gearbox, etc
Drove a 65 gold bulldog, 300 plus 5X4 two sticker yup a front to back Mack. Thing had about a gazillion miles, but pulled hard drove well and shook the teeth right out of your head. I remember standing on the thing in soft not loose or slippery dirt. It was pulling so hard you could count the cylinders firing. Then it learched stalled and started running backwards. So I shut it off refired everything good except air cleaner full of soot. When you’d load those old girls with the machanical rack they’d blow black curly Q’s out of the stack. Ah fond memories