The Trek 400 is a nice frame and might make a fine conversion. The thing to check before proceeding is tire clearance. Will it accommodate the size tires you want to use? In my experience older steel frames with the sort of sport-touring geometry of your Trek make fine gravel bikes if the tire clearance is sufficient.
Here's a couple photos of conversions I've done. The first one is an early seventies Motobecane Le Champion. The second is an eighties Diamondback Hybrid.
Brent