To Spring mount that antenna you will have to remove the existing mounting bracket and make another one that is flat. No bend. The new bracket needs to have 4 holes. 3 matching the 3 that attach to the Unit and you need to have the plate extend out one side a bit and drill a 3/8 inch hole (or about a 1/32nd larger) there.
Now a 3/8 Fine thread as I recall bolt goes into the large spring, and it mounts with a 3/8 Fine stud to a new mount. (Ball type) you will need to get at a CB shop.. That's for the big spring.
Small spring 3/8 becomes 1/4 as I recall.. but check the spring.