Some guys use the roof metal from a van or old station wagon that has ribs in it for a ribbed look. I like them smooth, I filled the roof on my 30 Ford coupe, I just went down to a steel sales warehouse and bought a piece of sheetmetal about a sixteenth of an inch thick and cut it and shaped it and mig welded it in, hammered my welds down the best I could and smoothed things up with the grinder and a little Dyna-glass. For reinforceing you can use small square tubing on the inside your headliner will hide all of that. However you do it tack weld it in at first and then weld between the tacks at random switching sides etc, blow on it with air hose between welds and wait a few minutes for cooling in between to keep heat warp down. Just don"t try to rush it and you will be fine.