70 Duster ignition issue

When you say "holding the key in the on position" do you really mean start?
First thing I'd check is whether there's voltage at the ballast with the key in "on" (not "start") (check both sides of the ballast...if you have 12v on the ignition side but not the coil side you may have put in a bad one). If there's no voltage start tracing back to the ignition switch. Could be bad connection thru the bulkhead, could be bad connection at the switch.