Normally the battery grounds to the block and then to the body. So the current has to travel through a solid block to get to the starter. Whether it runs through an iron block or a steel chassis is neither here nor there, what matters is the connection between each. Typically the ground from block to body was a much thinner gauge than from the battery to block because the starter needed that heavy gauge for the current draw. So grounding first through the body and then to the block you're going to have to use heavy gauge at both ends.