[franken]

Current capacity of a cable is based on its diameter, not length. As explained earlier the exact same current flows into and out of the battery when cranking the engine. You can experiment on yourself. Breathe in and out quickly through your nose. Plug a nostril with your thumb. Breathe in and out quickly through your nose. Which works better? If current capacity was based on length we could theoretically place the battery a few inches from the starter and run AWG 12 wires. Nope.

Moving back to the possible heat issue, maybe get some heat wrap and wrap the headers in the area of the starter?

As far as batteries go, I measure the battery box, decide on terminal location and buy the biggest battery I can find. I also expect them to be toast in 4 years.