Won't start in Park (73 Charger)but I can find a "sweet" spot in Neutral after a few jiggles (center console)that makes a click, contact, and then ignition works as normal. My question is : can I make whatever adjustment needs to be made from "topside" or will we need to adjust underneath on the trans itself ? What am I looking for ?

The neutral safety switch is on the drivers side of the trans above the pan rail.
I like to avoid all that stuff and simply ground the brown wire on the starter relay. This allows me to start the car in any position. For a new driver or an idiot, it may present a problem though.

Put the shifter in park and adjust the linkage AT THE TRANSMISSION. There is no other adjustment.

BUT, make sure that the shifter is bolted tightly to the bracket. That could cause an issue.

If it does not reliably start in neutral. I'd start by replacing the switch.

If I read that wrong and it's only a problem in park AND all the gears work correcting relating to the shift indicator.
It may be worn linkage. After years of use some of the pins wear, become smaller. Also the holes wear and become oval. So the linkage can not push the transmission fully into park. You adjust to compensate, then you can not go into 1st.

Bypassing is bad advice.

And don't be surprised if the 3 wire plug on nuet safety switch is rock hard, non-cooperative, cooked piece o crap. You can splice in a new one.

The problem is a linkage issue which might be need for adjustment or slop caused by worn/missing plastic bushings in the linkage.

Crawl under the car and have someone run it through the gears, look for slop in the linkage.