Racing in 100+ heat, any advice?

Stand 21 makes rather pricey but excellent breathable (like a cotton shirt ) drivers suits - huge improvement over anything else on the market.

Hydrate the day before. At the track drink water & maybe a little gator-aid.
Also eat healthy the two days before racing, you'll need carbs at least two days before you race.

As others said use a cooler with a lot of ice to bring your body temp down. Cold towels on neck & arms (wrist ) can make a big difference.