Ted, please document what you do and how you do it, with photos. Some of us are keen to learn.
That said, the thought occurs to me: the main disadvantage with a fiberglass part is that you can't bend it just a little for that final fit. My dad was a body man, and when I watched him work, I noticed he would adjust the hinge mounts first, then the striker, and fettle that a few tries until it was about as good as could be got that way.
Then, for situations like yours, where the front line was about perfect but the back was out on one part and in on the other, he might use a 2x4 or other stick, wedged strategically between two metal parts, to re-shape or twist the whole door just a hair until it was close enough to perfect.
Keep in mind, too, that in normal cars you'd be lining off the quarter panel, back to front, since the front fender was easily shim-able (and bendable) to get the front gap perfect.
School us!