Strange, the catalog lists one for 3/4 and 1 ton, but not for 1/2 ton. S
On to the problem. Seems to me that either there's one of 3 problems-someting's bent (control arm/frame/spindle), you have a weak spring on that side, or the mechanic just didn't want to mess with a 40 year old truck.
You might have another shop check it out, just to get a second opinion. Have them check the ride hight and compare left ot right.