Hi Dave,
I don't work with fiberglass, but I'm told epoxy based resin will do a better job attaching to steel than regular fiberglass resin. As far as the gaps go, do you have any pictures? Sometimes you can trim pieces to fit better, other times the contours will not match up as well if you trim, so you have to fill the gap instead. Use a flexible machinist's ruler or equivalant to bridge the gap. This will show the contour, and give you a better idea what to do.
http://metalshapers.org/cgi-bin/community/communityalbums.cgi?ownerid=9990093068868
John
www.ghiaspecialties.com