bottle shouldn't be mounted sideways... recommended way to mount bottle is with the top facing the front, and the butt of the bottle facing the trunk, with the bottle at a 15 degree angle (front higher than the back). This assures taht there is constant feed of n20 when it is engaged. It won't blow your engine or anything, but if not enough n20 is sprayed you'll just run really rich and it will feel slow.
I was going to get a n20 system put in, but eventually decided against it. But I woud have put it in the trunk, on the far left back corner or far right... PLUS if you want to use it "safely" on a track you have to install a blow-out/off tube that will vent the n20 out of the car in case the seal bursts and there is a leak. Guess you could install that tube behind the seat too, so that's not a problem anyways.