I would think that unless the vehicle is pulling left while in motion, the warn wouldn't have much to do with the braking of the truck. I would inspect your factory 34 year old drums, cylinders, and shoes, and if they're good, I'd try adjusting the brakes and then see if the problem remains.
I had the same probelm. Check your steering arms!! One of mine were loose and moved when I braked with the original drum brakes!! Mine pulled to the left (really bad way to pull here in the US). My USDS steering arm was loose and allowing the knuckle to flop out to the left and cause the steering pull.
Pretty dangerous stuff. Glad I found it after beating my head against the wall trying to figure out my drum brakes, then deciding to do the MAF disc conversion (which I love BTW), and starting on the PASSENGER side that didn't have a loose steering arm. So, I had half the kit installed when I figured out I really didn't need to convert to disk brakes due to the pulling.....
You don't have to say it... I did feel like an idiot at the time. But I don't regret the MAF disc conversion. Has worked very well all this time.