I install CBs and antennas regularly.
I have a 2002 Freightliner Sprinter with the high roof (over 6' inside) and have a wilson 2000 mounted on the roof. It's only 4-5' tall but it's on top of the roof and it's not too high. Doesn't hit bridges or anything. I even went through McDonald's drive through but with the antenna mounted in the center of the roof it was actually just outside of their roof so it didn't hit. Otherwise it would have.
The center of the roof is the best place, performance wise, to mount the antenna.
If you are still absolutely against the roof, you can do like another mentioned and fabricate a bracket that comes up through the crack between the hood and the fender. If you need mounts or any information don't hesitate to contact me at
aaronduke@email.com