Converted 74 Duster to auomatic on the floor, and now won't start. HELP!!?

Probably the neutral safety lock out! Check out the wiring diagram for your car and trace it back to the starter solenoid. Then jump that terminal to ground and if it starts, bingo! Then find the wire from the safety switch on the transmission and make sure it gets back to the solenoid. Your shifter linkage does engage the neutral safety switch- right?

If you don't have a manual, go here for a free downloadable service manual. MyMopar - Mopar Forums & Information - Service Manuals