Best bet for scale would be 1/35th. If you are building the whole base, it would be pretty huge. As you can see above, it is huge. The way they are constructed is with the larger caliber weapons on the inner ring and smaller weapons as you went out. Some had from M110 8" howitzer all the way down to MG pits on the outer ring. Depends on what you want to make. I would suggest you start with a section or one gun emplacement, unless you have lots of room. You should be able to get all the kits you need in 1/35th scale. Here are a few that would work.
M109, M110, M1 howiters all by Italeri
M102 Howitzer by AFV Club
107mm Mortar by Tamiya
Heavy Weapons and many Vietnam Figure sets by DML
M151A1 Jeep and M113A1 by Academy
M48A3 Tank by Tamiya
Mechanical Mule by DML
The list could go on and on.
The layout of the firebase was usually on high ground that was easily defensible from all sides. All weapons pits were connected by a massive trench system. There was usually one entry/exit point that was controlled by a sentry with MG nest nearby. The hiltor or area the Firebase was constructed on was cleared of all vegitation to allow good visibility and deny the enemy any hiding places. Hope this helps. Happy building.
Gino P. Quintiliani - Field Artillery - The KING of BATTLE!!!
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