Im roasting in my car, ideas to cool it down.

66Valiant528 said

right against the firewall. I was thinking about like reflective heat shield stuff. Inside on the firewall.

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Any room at all to get sheetmetal between the header and firewall? Any bit of an airgap between the header and a shield, and the shield & firewall would be substantial. The firewall metal is going to act like a heatsink and spread the heat over as much area as possible.
Add a duct from the grille area to "spray" cool air at the area of firewall that the header tube heats up.
I know its a race car, but create a headliner out of 1" fiberglass ductboard. use a small paint roller to roll resin on the cut edges to prevent flying fibers. it doesn't weigh that much.