mine wouldnt fit very well with it in that position so i installed it horizontally front to back up on the side of the wheel well. spoke to the man at firetec and he said it was ok to do so.
also i couldnt mount it vertically next to carbs as the brackets top mounting hole was on the seam of the arch, strange, however i did notice from other photos that my bracket was different to everyones elses
The horizontal and vertical are identical all you have to do is ensure the black dot is lower most, the roof mount is different in that the internal pick up pipe is positionedin the same orientation as the valve so you wont be able to fit above.....