[lolife99]

Your truck came from the factory with a wider front track width.
What compounds your problem is the 67-69 (and early 70 model) trucks had a narrower rearend than the disc brake equipped 71-72 trucks.
By someone adding disc brakes it makes the track width differences ever worse.
But all the 67-72 and 73-87 came from the factory with a wider front track width.

This is why many people run a wider rear wheel/tire combo on the rear to help hide this issue.