I don’t have the clearance as some do but could prolly get them pointing towards the cab.
Yes, the noisy end of the horn comes from the trumpet.
They are directional.
My whistles are great because they are unidirectional, providing 360° coverage. I’ve used them multiple times for alert behind.
No matter which direction my bells face, I aim them downwards the best that I can. Obviously whatever you install them on can limit how you mount them, but remember that sound reflects. 
Mine are on the frame pointing straight down.
I’m heard well over a mile away.
If you want to be heard farther than that, mount them high.
-Kris