Yeah, sounds like not enough positive caster; positive caster will tend to return to straight-ahead, or "centering", and increase the amount of effort required to deviate from straight ahead.
I'm assuming no worn-out parts, and camber & toe-in are OK or close, nothing really weird at rearend or offset wheelbase.
Basically, older Mopars are positive-caster-limited by their architecture; so you can't really go too far positive unless it affects the camber adjustment.
1/16-1/8" toe-in, 1/2-1.0 degrees neg camber, and as much pos caster (3-5 degrees being the usual design limit stock) works.
(My drag car runs more pos caster than that, and centers with a vengeance - goes dead straight downtrack, which is its narrow purpose.)
Mopar Collector's Guide is in Baton Rouge - they own some Mopars, and someone there may know an alignment shop in your area.