pushbutton vs later wire harnesses

I would guess the biggest difference is the reverse lamp switch. The neutral switch is a single post switch on the trans, and you would have to ground the starter relay if going auto to stick. In the opposite scene, just use an auto start relay and run one wire from the relay, down the firewall, to the trans switch

The reverse lamp switch would be on the shifter somewhere, and a stick is on the trans, so you 'd have to deal with that.

Wander over to MyMopar and download the closest you can find for a service manual. You might have to settle for Plymouth vs Dodge. There are changes through those years, so you'll have to be on your toes, but these are simple cars