The harness I installed is the "classic update" harness from American Autowire. Very high quality, but also pretty expensive.
1974 DODGE DART American Autowire 510603 American Autowire Classic Update Series Wiring Harness Kits | Summit Racing
It's also designed around the stock wiring layout with exception of the fuse block which is moved. Might not be what you're looking for, you may want something a little more generic if you're planning a push button start and other modifications. It also requires a good quality set of crimping tools, although, if you're going to do a custom harness you will need those anyway.
The instrument cluster doesn't need to be plugged in, but the ammeter does need to be hooked up or bypassed.
