1st. gear on that transmission is designed to get a heavy load moving. It is not a regular road gear. The SM420 transmission does not have a synchronized 1st. gear, so the vehicle must be at a complete stop before engaging it, or severe grinding will result.
If you're planning to replace the 8-lug rear end with a 5-lug axle, you'll also need to swap front hubs for ones with the same 5-lug bolt pattern. There are plenty of options available for higher rear end gear ratios, and you could use a 5-speed overdrive transmission, or a Muncie or Saginaw 4-speed with an appropriate rear end gear. You'll be basically turning your truck into a car by eliminating the load-pulling capacity of the 3/4 ton drivetrain and the weight capacity of the heavy-duty wheel bearings.
Jerry
