Slow Cranking

Is the generator working properly? Does it only do it when the engine is hot? It sounds as if the starter is suffering from Heatsoak. Take it to a shop that has a VAT40 or equivalent machine that can test the charging and starting systems on the car. The starter should not draw much more than 100 amps and the battery voltage should not drop below 9.5 while cranking.

This sounds to me like heat soak. You can buy those sleeves for over your starter but if I were you I would remove the header and use some header wrap around the area near the starter. That's what I did to protect my cable clutch from melting.

Hi,

I'll add, if "heat soak" is suspected after running the other tests, you might consider installing a small base starter. It provides a little more distance between it and those headers. The ceramic coatings are very good at keeping the heat in those headers, as well, and functional at the same time.
Also, adding a insulating wrap to the header in that area may help. But, I'm not a fan of wrap with the discovery of ceramic coatings. I've been running a small base starter, with my ceramic coated TRI-Ys for a couple of years without any heat soak related problems.
Good Luck!