I'm not sure on A bodies, but B and E bodies with 4spds did have linkage to the column in 1970 as you had to put the car in reverse to get the key out. I would start isolating some wires, check the harness(all of them) to see if they have rubbed through somewhere, been piched, aftermarket...
I know the heat range was 10 for most of theMopar muscle cars that were over 10:1 compression.
RN11YC were used in the mitsu 3.0L engine that had 150-160 psi cylinder pressure so I would think they would probably work well for your application.
Power comes from the fuse to the brake switch, when the brakes are applied power flows through the switch to the turn signal switch and is re-directed to the rear bulbs.
Your hazards work?
Checked power from your turn signal fuse?
Make sure you load the circuit with a lamp to make sure you...