For the most part the engine harness is all you really need. I just didn't want to go the effort of modifying the dash harness, plus we need the OBDII connector to pass smog. This was just easier for us. We also used a 92 Ram instrument cluster for the electric speedometer. Since you plan on running the 904 you will need to change out the speedometer drive. We did the engine/trans swap together as I really wanted the overdrive.

Whatever year engine/harness/computer you are using you will want a wiring diagram for.