Jerry gave you good advice. On the driver's side of the transmission are two levers. One actually shifts it into the different ranges and the other is the throttle pressure lever. The factory adjustment uses special tools that lock the carb linkage and adjust the pressure lever in reference to the back of the transmission main body. You can dial it in by trial and error as Jerry mentioned. The rod goes through a swivel with a lock nut on either side of it. You loosen one nut and tighten the other to make the adjustments. An eighth to a quarter of a turn makes a huge difference. You can go from shifting all 4 gears in about 3 feet to the other extreme of winding way out between shifts and not downshifting when you slow down. Look through the old car manual project set up bu Keith Hardy and you will find the factory service booklets for adjustment and maintenance for 54 Chevy trucks.