Please help!! 1971 340 duster having major ignition wiring headaches.

Question: after it started and ran did you set the timing and tightened the distributor down, so it won't move?
The distributed was tightened down before starting and is still tight and don’t move. Also it ran and idled very well so it seemed the timing was spot on. Would any of that cause the readings to be off and the Volts on the +side of the coil to be reducing to 4V when trying to crank instead of reading the 12V I believe it’s supposed to be showing when turning the key to crank?