Big Issue on harnesses from Classic Industries
What I have found is that the deviation from the standard package harness occurs with convertible and with cars that have been modified over the years. Otherwise, it's pretty straightforward. Back to the basics of wiring diagrams as a start point.
There was a lot of modification for the engine harness with fan relays and addition of electronic ignition. The dash was simply adding the parking brake indicator (period correct) tach and under dash gauge cluster addition. I also added a fusible link... That was simple plug and play