If you want to upgrade your 64-66 to some accurate aftermarket gauges, there is now a reasonably easy and cost-effective way to do it. Total time involved was around 6 hours, most of which was spent making up the wiring harness on the gauge set.
The key to the whole thing is the aftermarket gauge support (Part Number GS). It's the black vacuum-formed plastic piece that holds the aftermarket gauges in place.
You need a 1965 GT or 1966 instrument bezel - available in either black or woodgrain. Gauges are up to your personal preference. There are plenty of choices in various colors and styles (white, carbon, cobalt, etc.). You could even go black and look close to original with MUCH better accuracy. I went with white-face Autometer Phantoms - fuel level, oil pressure, 160 speedo, water temp, and voltmeter. The other two gauges, tachometer and vacuum, are mounted in a re-pop Shelby dash pod ($5 swap meet bargain).
The wiring harness is tidbits from Radio Shack. Two sets of 9-pin male-female harnesses. I can unplug the entire harness with two plugs, unscrew the speedo, then pull the the bulbs for hi-beam (left side) and turn signal (right side) and it's all out. I did a mix/match with the male and female plugs. One is male and the other side is female, so you can't plug it in wrong or backwards.
The dash metal did NOT have to be cut. I simply trimmed one little piece from the bottom of the plastic bezel, and it all clears fine. I did bend the metal tabs 90 degrees so the bottom screws go right in.
Good swap all-in-all. Difficulty factor is around a 4 on a 1-10 scale. Improvement factor is a 10.