Ditto what RDJ said. It will make life much easier if you loosen the bolts on the steering column base plate at the fire wall and the two holding it up at the dash. Let it hang down to give you more access room.
If you disconnected the speedo cable and wiring for the light switch and windshield washer knob, the only things remaining should be the two wires for the ammeter (upper left corner) and the main harness connection to the instrument cluster (several terminals in a round plug, just to the right of the high beam indicator light). There are maybe three screws on top and three on bottom holding the cluster to the dash. Take those out and you should be able to coax the cluster forward enough to get at the main wiring harness plug.
As for the manual...
I bought a Factory Shop Manual for 1974 Dodge from Detroit Iron Information Systems, Inc. (www.detroitironis.com) for about $49.
The heater core is located inside the underdash air box. If the previous owner removed the factory A/C, did they replace the air box with a non-A/C style? I bought a new core at my local Advance Auto. The only problem I had with it was that the two inlet and outlet copper lines were bent at an angle slightly different than the original core. They did not penetrate the fire wall correctly, so I CAREFULLY bent them to the correct angle. You don't want to break loose a soldered joint.
Jerry