can you see any movement on the splined steering shaft coupling when you pull on it near the box,there is a collar with a nut and bolt which keeps it tight,,??
It may just be seized up,, after all it's a 1958 thats a long time to be tight.
My memory tells me they where a relatively simple affair ( first truck my dad bought me was a 58 GMC fleetside, many years ago),,,i'm thinking the shaft is just siezed.