Car wont start unless door is open

There's an old old thread on here about that. Of course start with your service manual
How To Install an Ignition Switch in 1970 Dodge Dart

However, I would not replace it until you make electrical checks. One thing others have pointed out, is that those switches sometimes get "loose" inside, and the contacts -- pieces can "wander around" inside.

Learn to pull the connector terminals out of the switch connector. Not that hard. Remove the ones you don't absolutely need, and get them down to ONLY

the power coming in
the "start" wire
the "run" wire
the "ignition bypass" wire

"Safe off" the rest and see if it works with just those only.
If so, then re-insert the "accessory" wire and re-test

If that is OK, re insert the others one at a time and retest.