Electrical , Not cranking by key

Then it's either bad NSS or out of adjustment, or problem in the "rooster comb" internally

Make certain switch has the correct gasket (I can't help you there) and is properly tightened

If you are shifting by hand it might not be engaged. Try N instead. Your mention of the seat belt deal interupts the path of the yellow wire on the road to the starter relay, but that only affects 74 cars, and they have the infamouns "reset" box under the hood