Regarding the fiberglass. We do not pre-drill holes. Our recommended procedure is to clamp everything together then drill holes, that way they always match. I prefer to drill holes 1/2" diameter for 5/16 gr 5 bolts, this gives a bit of wiggle room (stock 3/8 bolts can be used also). Our recommendation is to use USS hardened washers on 5/16 bolts, stock thick SAE washers with stock 3/8 bolts. With proper washers overtorquing is not an issue (and the oversize hole prevents binding of the bolt shaft). Felt between panels may be used but does not need to be used. Fender welting is discretionary.
As to the body and frame mounting there are wood blocks for the 2,3, and 4 bolt, and front bed bolt locations. I recommend a treated fabric chassis welting on the frame from the #1 bolt hole to the front bed bolt hole.