Inside vs. outside uneven wear on the front tires often indicates a front toe in or camber problem. (Googgle "home wheel alignment" for links that detail how to check the front camber, caster and toe-in).
Are you certain that the uneven pattern/wear is not a result of improper front toe in on tires that have been rotated from the front to the rear?
On the rear (especially with an AMC 20) it is a possible indicator of a bent axle housing. To check; level one tube of the axle housing and check the other tube if it is not also level then it is likely bent. You can also use an angle finder or level on the axle tubes and outside of the wheels to verify that both are at 90 degrees (vertical) to the axle center line.
In answer to your question; it is unlikely that tire pressure changes will affect this much, if at all.
...a well prepped, well driven, vehicle should do well in any terrain, including the highway.
Carburetors became obsolete during the last century... do what ever it takes to get fuel injection...It makes bigger grins off road.