A couple of more ideas or suggestions.
First, you need 1-0 or 2-0 welding cable for battery cables. The bigger the better. Six volt takes twice the current of 12 volts so you need the large cable to handle the huge current load.
Ground the battery right to the starter. Run secondary grounds from the engine to the frame and engine to the cab sheet metal. Clean the metal of all paint and rust, and use star washers to bite into the metal and cable end.
Charge the battery overnight, have it check if theres any doubt about weathers its good or not. Useing a good charged 12 volt battery will work for test runs. Be sure to shut off any lights and accessory's, 6 volt things don't like 12 volts. I would unhook the fuel gauge to, they really don't like 12 volts.
Spin the engine with the plugs out and motor oil in the holes. Not so much it sprays across the engine bay, just enough to lube the cylinders.
Change the oil in the pan if its old and dirty.
Have the engine full of coolant, be sure and burp the coolant system. Drilling a small hole in the thermostat lets the air out but doesn't effect the cooling. 216, 235's are bad about holding air in the engine.
Have fresh gas in the carb. Use a clean fuel jug with line going to the pump, bypassing the trucks fuel system. Once running you can swap it back to the trucks.
Have fire extinguisher handy, timing light, wet towel (to snuff out small carb fires), and some help close by.
