I mounted my tank underneath the truck in between the driveshaft and the frame rail. The compressor is also mounted to the frame rail and the horn itself we had to make a bracket for it but it is also tucked up quite nicely just behind the trans. cross member.
The horn I have installed is the Nathan K3. Nathan also makes a K5 but is has 5 chimes and won't fit under the truck as well. The K5 also doesn't seem to hit is hard. Don't get me wrong it wil still blow people out of there socks but it just doesn't seem to have that hard bang when it hits.
Try these sound bites:
Nathan K3
Nathan K5
More information on train horns
look here.
Want more info? Here is one for the
Railroad Airhorn Guide.
And finally one more helpful site that explains a little more in detail some of the different horns.
Found Here.
This site
here has a good diagram of how your system should be laid out.