1970 Dart starting issues- key will not crank now

You need to make voltage checks. Draw it out on a piece of paper. The path for the main starter current is From battery NEG post----NEG battery clamp---------cable---------engine block------starter bolted to block------through starter-------out on "big' starter stud------through main POS cable-------POS cable clamp---------to battery pos post.

You can clamp one multimeter probe somewhere, (or have someone hold it) say, to the battery NEG post clamp. Now while jumpering the starter, probe the battery POS post and read the meter. You want at least 10V or higher. Now you want to read voltage drop. Again, with the probe clipped to battery NEG post clamp, stab the remaining probe into ta good ground on the engine such as carb bolt. Crank again, read voltage drop and note. If you can get to it, stab your probe into the starter and read it there, the starter motor shell. This reading should be very low, a couple tenths of a volt or less

Now read the positive drop. Clip one probe to say, the battery POS post clamp. Stab the other probe into the starter "big' stud, crank, and read. Again no more than say, .2 (2/10's) of a volt

Measuring some of this stuff alone can be a challenge. Again first with one proble clipped to the NEG battery clamp, stab the probe right into the top of the NEG post. Crank and read. Should be very little reading, almost zero. Do same for POS post and clamp

Now check the starter relay. Clip one meter probe to the starter 'small' stud. Clip the other post to the starter relay 'square" terminal. Jumper the relay. What does it read? Should be almost zero

Now hook meter to a good ground, battery NEG clamp. Connect remaining probe to the relay 'square' terminal, and crank using the key. Should have 'same as battery' volltage. if not, move meter to relay "big' stud. Crank and read. If that terminal stays 'same as battery' the relay CONTACTS are bad, or the relay is not pulling in

Maybe it is not getting coil voltage. Disconnect the transmission neutral safety wire. Ground that flag terminal on the relay with an alligator clip. Connect a jumper wire to the remaining relay flag terminal. Make certain the car is "out of gear."

Engage the relay, and double check voltage at "big stud" and at 'square" terminal. If "big' battery relay stud reads 'same as battery" and if 'square stud' is low voltage and or starter will not engage, replace the relay.

Maybe it is not getting "start" signal from the key. leave the neutral safety grounding jumper connected. Replace the yellow 'start" wire connection. Crank with the key. If it does not crank, measure (back probe) the "yellow" start wire connector. If it is low, say, 10V or lower, you likely have an intermittent in that path to the ignition switch, which means bulkhead connector, ignition switch connector near steering column, or a bad igniton switch itself