I install a lot of doors for a local lumberyard. They can't get an off sized door of fiberglass as the panels are cast onto molds to determine the length. It is a solid sheet of resin and fiberglass scrim.
When you cut fiberglass, it is shatters and splinters. I tried cutting it, grinding/sanding, and all other types of removing length. No dice. The cut edge was OK at best, but when stained or painted it really showed.
The added problem also was that I corrupted the structural integrity of the door as I cut off part of the frame panel. Oddly, to make an off sized door of fiberglass, our local door plant charges just about the same amount.
In your case, you would have the added ugliness of a thinner side piece (stile) on one side than the other unless you are just putting in a flat door with no panels.
Robert