Also--if you can find it--I highly recommend the 1992 Bridgestone XO-2 or the XO-1 series. The bikes were designed to be fire trail bikes with road geometry and 26 inch wheels. My '92 b'stone XO-2 make a fantastic gravel bike.
Also very good are the bianchi project bikes as these were hybrids designed for really fat tires. Some of the early Trek hybrids (lugged) would also make terrific gravel bikes.