[LightningGSX]

From the factory, there is(supposed to be) a ground going directly from the neg battery terminal to the bolt that holds the starter to the bellhousing.Since you probably grounded the battery to the chassis in the rear of the vehicle, your starter is grounding through the tiny wire that goes from the firewall to the intake manifold.So, before you change your power wire, I would put a 4 gauge ground going from the bolt on the firewall(the bigger one where the neg term on the batt normally hooks to) to one of the starter bolts.That may solve your issue without having to change to larger power wire.I can go take some pics if this is confusing.

Edit: you may have done this already, where does the ground coming form the starter bolt connect to?