If you remove the prop, does the fiberglass sag back down?
If the answer is yes, and the windscreen is bonded in place, this will then place a stress on the windscreen, and will be pulling the screen corner downwards which will apply extra stress to the screen. If the screen is loaded, then it may only take the lightest of hits from a rock being thrown from a car in front to cause the screen to crack. The other problem is that the whole lot may sag over time changing the panel gaps and door clearances.
I would be trying to adjust it, so that it does not need the prop to maintain its shape. Either through applying heat, to get the resin up towards its glass transition temperature "Tg" where it becomes a soft plastic that can be bent (will depend on the type of resin used)
by cutting a slit into the A pillar on the tension side, the slit will then open up allowing the pillar to conform to the new position. With the roof and pillar propped in the new position, a scarf joint repair can be made to allow the slit to be closed. This should then hold the roof line in the required position.
That would be my thinking, and how I would approach it.
Will follow to see what others have to say.