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If they are all of the exact same quality, the bigger the better. But for a stock ignition I would just go with a set of high to very high quality 7 or 8mm's.
I'm running a MSD-6A and a MSD TFI coil, so I made sure to get high quality spiral-core 8mm wires.
Ideally you would get a set from a local parts store that are both of high quality and have a lifetime guarantee - then it will give you reason to create a special filing place for such receipts for your truck. (You can also buy lifetime guaranteed steering components and a few other parts - and well maintained straight-six trucks do occasionally outlive these things.)