Fiberglass does not adhere to steel very well. It gets air gaps then pops off, it will rust between the steel and the resin/cloth. The better bet is to apply it under the headliner for strength. You would have to roughen the steel to promote adhesion.
There's other tricks to keep the roof popped out. Once the roof is roughed in to shape, you will apply tension to either side of the top, I used this method on a coupe with a 2x4 along the rubber strips areas. If you use a torch to shrink the metal make sure you know how it works before you do it. Watch a video or maybe look on You Tube. Hitting a roof with a torch is like sandblasting the body, you may not like the result.
Miles