Anyone done an engine-side wire harness? How difficult?

You CAN rebuild them if you take your time and dedicate.

The common terminals like the bulkhead connector, and windhield washer use what are called "Packard 56" terminals, which are available here and there. I got mine at clips and fasteners.

Wire is available from several sources, one is "Del City" (No it has nothing to do with me, LOL)

The challenge are the "odd" connectors such as the temp connector

The big white plastic engine connector needs to be eliminated. That was a real bad "engineering" idea.

Unplug the harness, lay it out on a bench / board, and nail it down and tag / mark the "break outs" of the different wires. Unwrap the harness and tie it at the breakouts so you can match them.

Then you can simply do a "wire by wire" duplication. You can ALSO eliminate 'stuff' you might not need, IE you converted ignition, or went to a "one wire" alternator, whatever. Smog stuff, etc.

You ALSO might consider at this time making mods to add a relay, triggered by the key, to operate underhood ignition loads. Low ignition / VR voltage is the number one reason for overvoltage in these girls

And read the MAD article to decide what to do with your wiring around the ammeter/ bulkhead.