It is generally recommended to stay away from drilling/bolting directly into the frame member. In the engineering world, that hole creates a "stress razor". The hole is now a focal point for stresses in the frame and cracks will likely occur across the hole.
Much better to use U-bolts or angle iron tabs and bolt through the tab. In either case, on the top side use large fender washers or a large plate steel under the nuts to distribute the holding force over more of the fibreglass.
I added two square u-bolts near the front of my Boler
as the factory screws were rusted out. Those u-bolts saved part of the day when my frame broke near the front. I'll post pics if I remember how, haha