Since the length of the drag link isn't adjustable, there's another possibility. That horseshoe-shaped steering arm between the front of the drag link and the left spindle could be bent. Lots of potholes and other road hazards over the past 60+ years can do unseen damage to steering system components. There's a checking procedure for finding bent linkage that involves checking the geometry between the center of the rear axle and the pivot point of the kingpins that's a bit too complicated to describe in a forum post. Very few alignment shops, and even fewer vehicle owners these days have a good understanding of the physics of a straight axle steering/suspension system.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!