I have seen them mounted on the back side of the firewall, and I have seen them mounted in the holes drilled in the firewall. Either way works. Some have a small square holed drilled in the firewall under the mount to make a place for the spring on the nose gear to go so it isn't hanging under the plane. It all depends on the plane. You can pretty much put it any place you want. It will not hurt anything. You could also put a 1/4 inch ply plate behind the engine mount to move it forward a little bit if you need to, but that would be the last thing I would do unless it is totally necessary. If the kit was designed for it, how do they install it? That would be the best place to start, but you can do anything you want and it would work fine.