I would run a 1/0 wire. Also put a 300A ANL fuse inline on the positive cable close to the battery for protection, take another 1/0 wire and go from the negative terminal on the battery down to the frame and then add another same sized ground from the frame up front to the engine block. Make the connections to the frame and engine onto clean bare metal spots and nut and bolt the connection using lockwashers. I would also try and solder on the terminal ends if possible and route the cabling for the least chance of piching and possible shorts.
Also check to make sure you have a cable or large enough wire to connect between the battery or engine block to the chassis of the car so things like your dome lights, headlights, turn signals, etc will work. The frame and engine are not really tied to the body of the car and if this wire is not installed you may have dim headlights or you could pissibly fry a shifter cable.
Also put the battery in a vented battery box to vent the fumes to the exterior of the car (do so even with the Optima sytle batteies as they do have pop off valves on them and can leak corrosive fumes).
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