All of the above is good advice. First of all it is a 24 volt system. Get yourself a volt meter.
Wear gloves and eye protection. Do not breathe the dust in the battery box. Batteries are dangerous and can explode when under load, or if there is a loose connection/short in the connections between the batteries.
Replace batteries in pairs; if one is bad, then it will try to equalize the other battery in the circuit, thus draining the energy from the entire system. Dropping to a 2 battery system simplifies things and works well depending on your environment(think cold).
Be sure the slave cables get connected correctly, the center pin is positive, and the outer is ground.
Get red electrical tape and use it generously on each positive cable so you can easily identify which cable is positive.
Clean your terminal ends and make sure they are securely fastened to the battery posts, sparks from loose connections here are dangerous.
If you decide to install a different type of battery in your truck, be sure to check for clearance/contact on the tops of the batteries when lowering the seat lid. Secure your batteries to the truck well, whether with a homemade bracket, etc. Place a rubber sheet over the top of the batteries to insulate them from the seat lid.
Consider installing a keyed master disconnect switch in the negative side of the circuit, which will give a level of safety and allow keyed control of the vehicle.
Carefully inspect your battery cables, where they pass thru the battery box in the floor, and in between the cab/engine block near the starter. The A0/A1 truck tend to rub the positive battery cable in the area behind the starter on the edge of the engine block.
