As Randy said, the underdash harness is different for the gauge style, but the underhood feed is also different.
The repro underdash harness runs about $400 last time I looked, and the gauge feed is another $30-$40. Painless's new replacement harness replaces everything in the car, and runs about $465.
That said, I pulled a '66 gauge style harness out of a junkyard and cleaned & retaped it for my gauge setup. I went to a 6 hole R style with Autometer gauges. All I bought was the underdash harness to match the '66 harness.