I assume this is a coupe or a 2 door sedan. Are you putting in a "hard" headliner? If so, yes, don't use the metal bows. You don't want to use solid wood because it could warp. You don't need two pieces of 3/4" plywood, that's overkill. One piece will do, but use hardwood plywood for better holding power when you screw on the luan headliner base. After you put the new wooden bows in side to side, connect the bows front to back to make a grid to hold up your headliner base. Look at the '47 Ford in my project journal. The wooden gridwork was so strong I could literally hang from it. I screw metal "T" brackets to the ends of the hardwood plywood bows and attach the "T" brackets to the sheet metal around the perimeter of the roof. You can also glass them in or use other fillers if you prefer,... anything to hold them solidly.
No one lives forever, the trick is creating something that will.