Correct paint color for a 1970 Dart 8 3/4 axle assembly

Definitely Semi Gloss Black. If you have a compressor and spray gun, get some semi gloss black with hardener and spray away. If you do not have spray equipment, I would highly recommend Eastwood's 2K Ceramic Satin Chassis Black paint. The spray cans have a button on the bottom that, when smacked, releases the hardener inside the can. The result is a high quality, hardened paint that looks great. The cans have a great spray pattern too. I do have spray equipment, but I have used the Eastwood chassis black before. It works great.
2K Aero-Spray Ceramic Chassis Black Paint – Eastwood