A 17' antenna probably means your running an HF rig in the truck although 18' would be the length of a AT-1011. There's no mention of what radio Katahdin wants to run in his 109 so the answer was meant to be generic. The typical VHF antennas used on military trucks (1729 & 3900) are about 10'. 66" and 52" respectively. Because of the militaries need to move quickly they usually don't have the time to dismount their antennas before taking off so they began looking for practical places to mount their antennas. Inside the front bumper was convenient and relatively safe and they could be pulled down quickly. And it was a better option than mounting them on the engine side cowls.
The location near the top of the box is idea for propagation but not necessarily the best for motivation.
The other issue is that the 109 boxes have a tendancy to rust out when holes are drilled in them. Most of the trucks that got released before this last batch had terrible problems with rust. The last batch, which are the best, were released because they were replaced with newer model trucks like the Stewart & Stevensons and tended to be the best of the older trucks that units held onto until the very end. Adding holes isn't a good idea unless you understand proper rustproofing techniques to prevent it.
Kurt
KG6KMJ