Keith, The SM 420, the "Granny Low" as you know it and "Granny Gear Trans" are all the same trans altho those terms are also applied to others out there, like the SM 465 Trans.
My old '64 C-30, 6-292 also had a SM 420, but I would still use 1st on the road even when empty. I would just use it to get the truck rolling a bit, then shift. I'm from the school that believes in keeping very little load on the crank, plus I like to shift. You can change the rear like mentioned above, but the only reason I would possibly want to change a rear is because of top end speed. How are your RPM's in 4th gear when the truck is going about 60 MPH?
