As regards a place for mounting a C.B. arial.
By far the best place (from the point of view of transmitting as far as possible) would be as high as poss, and slightly towards the rear of center of the roof.
Although I appreciate that this might not be the best, or most practical as far as off-roading is concerned, it will throw an even and strong signal from this possition.
It will also be easy to S.W.R. in this position, having the largest and most un-interupted ground plane on the entire vehicle.
my 2 cents worth f.w.i.w.