1972 dart - stopped running while at 50 mph ? no warning

further update . today i tested the coil disconnected from everything , out of the car . both primary "sides" ( post to post ) they read 4.3 ohms in the 200 position of the multimeter . the secondary ( post to center tower ) read 8.25 with the multimeter in the 20k position . since a manual and someone on here said it should be between 1.34 and 1.79 primary and 8 - 12000 secondary . i decided to test a spare coli i have . it tested 1.8 ( 200 position ) primarys and 9.09 ( 20k position ) secondary . so i put that spare coil in the car . again no start , but this time i could not get the coil to spark at all hooked up normally ( i think it did previously with old coil ) but it did spark with the coil hotwired and attached to the spark tester ( set at 3/8 inch or more like 1/4th inch ) looked pretty good , but too bright outside to see it too great . then i did the same with the number 1 spark plug wire and did not notice any spark . but maybe did not wait long enough . after this i disconnected the wires ( firewall and radio static canister thing ) from the coil negative and hotwired the coil again and this time got spark from both the coil ( again looked pretty good , but again too bright outside to see it great , but had thin long sparks that would occasionally go orange ) AND a tiny blue spark at the spark plug wire . tester set to .25 inch . but would only be once per few cranks , which i guess is because the rotor has to spin around again to that tower . still would not start though .

the battery showed 12.75 connected normally as it sits , dropped to about 10.50 ish when cranking . this battery is 3 years old , but have been cranking and cranking etc. for a week now . i was going to charge it tomorrow . not sure how that is affecting things .

i am starting to suspect the wire that comes off the firewall going to coil negative , might be another part of the issue . it looks a little corroded where the wires are exposed . can that wire be tested with a multimeter ? or powered test light ? ballast connected or not ? key on or off ? cranking etc. ?

i cannot yet check the timing chain because i don't have the right sized socket , or breaker bar , yet to rotate the crankshaft . so , unless theres another way , it will be some time before i can try the test AJ/FormS suggested . i did try the other no spark test he suggested when the old coil was hooked up , but did get tiny sparks from the plunging screwdriver to the points arm and plate and the near grounded wire . what does that mean ? does that rule out anything ?

as some have suggested , i was going to try a different cap , rotor , condenser etc . as i have extras around here , some unused , some were working when removed , but did not want to get too far ahead without some input from everyone , especially about that wire from the firewall , and what the coil is or is not doing . thank you all for your time and patience . i am unfamiliar with alot of this electrical stuff .