FWIW, my experience with the Hornet suggests it's not the size per se that's the problem but the way loading the pack affects the effective torso length. There's not a good functional connection between the bag and the hip belt, so When you load up the bag, it sags relative to the hip belt. This effectively makes the torso length longer.
I understand why you want the pack to work (I like enough things about mine that I kept it despite this key flaw), but if you mod it, it's not going to change the fact that the effective length will be load-dependent. If you decide to go ahead with it, do make sure you load it up with what you'll actually be carrying and model various weights you'll carry during your trips before deciding what length, if any, feels right for an acceptable range of those loads.