wiringharness.com is what I used, plug and play, zero issues.

One variation from the original harness that I didn't understand is that there is a provision for a neutral safety switch (two thick purple wires) near the pedal bracket. Since the '68 with manual had no neutral safety switch, I had to jumper this connector to allow the starter to engage. According to their catalog it is the correct harness for the application (floor shift, gauges, manual trans).

Before spending money on a new harness, why not find the short in the harness you have?