Do you do anything to reinforce the shell where the screws go through? Otherwise I would think that the screws that the spring sits against would just widen the holes in the shell they go through over time.
You go through both sides of the shell... and you use big screws. The best idea would be to use nuts on the other side of the screws. However, I never used nuts, and my crossbow has been modded for years now and there are no signs of wear.