I have a one and a half pound Halotron, the clean agent replacement for halon mounted just in side the drivers door on the floorboard. Doesn't interfere with anything and easy to get to. BTW, I work for major fire&safety equipment co. in the Tampa Bay area. I also have a 5 pound Halotron mounted in the bed close to the tailgate. If you can find Halotron, buy it. Comes in variety of sizes. Most of the small sizes come with a vehicle bracket.
Most Fire Equip. companies sell them. They are somewhat pricey, but whats your truck worth? MikeyP