[Sirrotica]

I've been through this decades ago in a 73 T/A. Don't try to use the frame as a ground, there is too much resistance in the steel that sucks up amps needed to crank the engine when hot. A hot Pontiac engine will need 4-500 Amps to spin over.

Run a negative cable all the way up to the engine same size as the positive cable attach it to a bellhousing bolt near the starter. It will crank plenty fast no matter how hot the engine or starter gets. Every configuration I tried using the car as a ground, failed when everything got heat soaked. Even with a commercial battery and a freshly rebuilt high torque starter (gear reduction mini starters hadn't yet been invented in the mid 70s).

That's my experience, and how I solved the problem, never had another slow crank after running a ground cable all the way to the engine.