I agree with Ted and Franz.  I'd get something like a 2 ft mirror mount

like this Newtronics Hustler Super Stick and mount it  where convenient. Mirror mount, light bracket, wherever is easy.  plus it is bright red like a good tractor should be

Your application is different than the hobbyist or truck driver wanting maximum range.  This type of antenna will take fairly severe abuse and be short enough to stay our of the way of overhead lines, etc.

On my CIH Magnum I mounted a base load FM business antenna on a piece of scrap galvanized channel iron I had laying around and used sheet metal screws to attach it to the fender outside the rear right corner of the cab.  It is not ideal from a purist standpoint, but it is adequate for our operation.