Here is a better view of my mount you were asking for. It's a Shakespeare 4245 120-31. Those have a 1" stud instead of the ⅜-24 standard. The antenna is a Hi-Q 6/160 RT MC3, pretty much the equivalent of the current Piranha 6R-.
If I were mounting an antenna on a vehicle I would not mount on the side at all. If you are going to be tight on space or if it is going to hit anything it will be on the sides. The Hi-Q is a very tough 15-pound antenna. Physically the antenna does not change length during operation and it is completely sealed on top. Your standard screwdrivers that change length or have two piece bodies eventually we'll get dirt inside and not to mention they are supported by an insulator. So between the normal screwdriver vs the Hi-Q antennas there is a difference in physical strength. So I'd recommend either front or rear mounting the antenna. You can probably get away with it being a little bit shorter if you went from the front. With any loaded antenna on a mobile you want to get the load in the clear, so above the roofline.
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