I am slowly working on something similar, but using civilian stuff. In another thread a member said that he is using a Sigtronics intercom system and that is what I am going to put together. The Transcomm II is portable so I can use it when I ride in someone else's MV or whatever. I have a similar broadband antenna by Harris on the truck now. One of the local ham's has a very nice antenna analyzer I would like to hook up and see what the SWR looks like on the common frequencies. What radio will you be running? I would like one that is waterproof or water resistant at least...my thought being it would be less susceptible to corrosion from salty sea air at the beach.
I wonder if I was that member. I used to use a sigtronics transcom system, the portable one. I have it still sitting here actually, I haven't used it in years. Used it with Jeeps, that kind of thing. I have an analyzer and the SWR on the Harris pretty much comes out to what they claimed in the specs, it's about 3.5 or better across the entire range. I work 100 miles on 2 meters without a tuner, 75 watts.
I plan to use a FT-857 with it, I have it mounted up already in the rack so you might see that in the pictures. I hear you on the corrosion thing but if the salt air in the cab is going to destroy a ham radio it's going to destroy just about everything else in the truck too, including a Sigtronics. Just a thought but I wouldn't worry about water-proof so much for the radios. The intercoms and military radios are that way because they are used out in the weather, and uncle sugar has all our money to play with so why not.
PS, that 857 banged around in a land rover for years, then a jeep, then rattled around in a go box, and it has been on the beach more than a few times. Just keeps going.