Yeah, the gauge lighting has always been low, especially on the 70-73s. Have you taken the cluster out and replaced all the bulbs, and cleaned the gauge face lens? Might help a little, but don't expect a dramatic improvement.
Mustang Monthly ran an article in Oct. '99 on how to brighten your gauges, but it involves a complete cluster removal, repainting of the gauge faces and bezels, and polishing the bulb lenses. I think the consensus is, if you don't want to put aftermarket gauges in, you pretty much just have to deal with it.