Trunk mount battery cable???

The problem had to be elsewhere. I've used welders cable on several dirt track cars & on a high compression 340 without any problems at all. I assume you ran your ground to the frme rail at the back. it's possible that you didn't have it grounded well enough or that the chassis to motor ground cable wasn't grounded right.

When you grounded your battery, did you grind the grounding surfaces clean?