My 74 Dart also had a few of the sockets broken and the bulb wasn't making good contact, therefore not lighting up. Go to Advance Auto and look for "CALTERM (08523) Instrument Panel Socket". I think it came with two replacement sockets in the package. It fits 5/8" instrument panel socket holes. Advance Auto part 5473136. Should be around $3.00 per package.
Check your original 74 owners manual. The speedo/instrument bulbs were #158. I think the 158s have been replaced by 194s. Sylvania makes a box of ten 194 bulbs (Sylvania part #34761). Advance Auto part 5081602. Again about $3.00 per box.
If you don't have an Advance Auto, call Calterm in Milwaukee, WI at (800) 228-5123.