Yes, no problem. I have rebuilt a number of '60's and 70's Ossa, Montesa, Bultaco and the odd 1960's Rickman that have a number of riveted fiberglass parts in a vibrating, stressful situation and none were an issue after 30 to 40 years. Done some fiberglass kitcar stuff as well using rivets for various things. Correct and probably not ideal, not because of contraction etc, but simply because fiberglass doesn't bond to metal permanently and one day it will separate, that's a given, the question is "when", not "if". That could be 100 years but I still don't like those odds. There's 6 ribs per side, after glassing as per instruction, pop a rivet in the second and third from the outside on the vertical face facing the rear (where the tube is in compression), and 4 rivets equally around the circumference on the outside one (number 6). Do not rivet the inside 3. That will be enough.