If we could just take a minute to think about "why" you shouldn't put the tank in the cab, I think that the apparent answer would be,
IT'S NOT SAFE!!
Let's say you get rear-ended, or you rear-end someone. That tank is head height and do you really think that it is going to stay there when there is a large amount of g-force pushing against it? Probably not, but it's your choice man...
Another big reason if the accident doesn't scare you is this:
Just think about it man...