[68c10airstream]

Welcome from across the pond,

As an example imagine if the pitman arm was disconnected from the linkage, gently turn the steering wheel all the way left, then right, counted the turns and found dead center, this would be your straight ahead position.
The next step is under the assumption that the gearbox has been mounted in the correct position, meaning basically straight ahead.

A good front end shop should be able to handle the alignment to straighten it out for you. The newer alignment shops actually have the feature to show what tie rod end to adjust and how far to adjust it to.

As an example to help understand the center position situation, mercedes benz in i believe the 70's had the feature that the rack and pinion (fewer moving parts than your truck system) had a bolt you remove from the rack and flip it around and reinstall it in the rack. The only position it could go in was the center of the rack forcing the mechanic to be in the center position to then adjust the tie rod ends for toe in/out and to center the steering wheel so when the alignment was done the steering wheel was centered when driving straight ahead.