Plug-and-Play Wiring Harness

The Assembly drawings will be a worthy investment for correctly routing and supporting the harnesses. its not a great reprint but will answer a ton of questions. Wires that aren't supported properly can cause failures over time. The connections get strained and the insulation gets damaged.

The assembly drawings also show how the connectors go together.
M&H is probably the best source but even they may not make all of the subharnesses.
Your interior'dash' harness may still have its p/n on it. Its by the dimmer switch. same with the body harness going to the dome and rear lights/fuel. Those may help when ordering.
You'll want to be sure the interior and engine harness are set up for the type of neutral safety and reverse light switches the car has. IIRC the switchover was late in the '68 model year.