You need to cut a slot into the box. If you just squash it in the seal will not fit around the bonnet correctly and apart from not closing properly it will leak water into the heater box itself.

I've recently posted two photos before and after showing of how it needs to be cut to fit properly. I'll see if I can find them as I'm not on my home comp.

As for fitting it: i used the self tappers provided (but cut a thread into the strut first using a tap & die set) altho most people say you should remove the suspension and properly bolt them in as this gives a more positive fitment. Depends how hard you push your car should determine if you want to bolt it in or if it's a bit more for looks than 100% function.