Some really great information above. My concern is the uni-body is not designed for starter current. Steel has about ten times the resistance of copper, for same cross section. .
That may be true, but this has been done by many, myself included. "In a previous life" the old 70 RR had a warrmed up 440, rear battery, and back then we didn't have mini-starters, so the starter drew way more current back then. This was a factory 10.X:1 440 Cranked over just fine.
There are some that advocate a separate ground cable, but the body can be and has been successfully used.
But you and I both know, "guessing" is not the way to fix this. Logical testing will show up the problem.