old Dodge, new problems

the neutral safety switch has the detent positions that completes the starter circuit based on position.
If you have messed with that lately it's probably the culprit. I believe the plug can even be put on backwards if i remember right.

probably the issue since you mention you already unplugged the seat lock.

unless of course your battery connection is sloppy. I remember grabbing an old battery out of whatever at one point, and the terminals were just undersized enough where it created connection issues.

Welcome by the way!
(74 swinger owner also)