Chevybruce, a few questions,
How much of the steering and suspension is new? tie-rod ends, drag-link, king pins, spring bushings and shackles, and springs?
What are the alignment spec's? toe-in and caster?
Setting still, with a very light touch on the steering wheel, how much play does it have before you feel resistance?
Have someone watch the tires, how much do you have to move the wheel before the tires turn?
Have someone watch the output shaft of the steering box, how much movement of the wheel before the shaft turns?
What type of tires do you have? How much air is in them?
What type of tie-rod and drag-link ends are on the truck? The original spring loaded ball & socket type are adjustable, have they been set properly? King pins been checked and greased lately?
With these questions, we can determine where the play is. With a correctly set gear box, in the straight ahead position, the output shaft and worm gear are making 100% contact, so zero play.
Jack the truck up so both front tires are off the ground, turn the wheel to the far left, then turn back to the far right, DID YOU FEEL it get tight as you crossed center? If not, you box has worn parts or is not adjusted correctly.
As for driving, how much caster angle does it have? Are the caster wedges in place between the springs and axle? Are they in the right direction, thick part towards the rear of the truck? 4 degrees is a good starting point. With out enough caster, the steering feels light and you are always making corrections. does it return to center after making a turn, to much caster brings it back really fast, too little and you have to turn it back to center?
I have a '41 gear box in my '37 truck, it drives as good as my '03 truck, no wondering around, straight down the road, one handed driving. Theres no reason to change yours, properly set up it will drive as well as new truck does. They didn't wonder around when new!
