If I were you I'd use a 90s 26" mountain bike rather than a 70s road bike. Yes, you can fit a relatively wide 700C tire in many old road bikes, but I personally would not want to be limited to 32mm or 35mm for a gravel bike. Much nicer to put drop bars on a 26" mountain bike and have the option of running 2" knobbies or slicks. Here's my gravel bike (1999 Schwinn Homegrown):
