Mitigating heat is an important part of building most cars, especially those with a mid-mounted engine, as in the SL-C. Some of the key areas to address are keeping heat away from the cockpit, insulating the shift cables, managing exhaust heat in a general way, and protecting wires, especially the engine harness, from excess heat. The table below shows some of the products used by other builders with success.

Product  Description 
Lava Shield Made from crushed volcanic rock with a trick carbon fiber look. Commonly used on the firewall.
Reflect-A-Gold  Metalized polyamide polymer laminated glass cloth with a high temperature pressure sensitive adhesive for use in extreme temperature swing environments. Plus adds lot of bling (see picture below)! Fling's build thread has some good info on how to apply here and Yos has some info here.

Sealing the car, especially where the body meets the chassis, is a key strategy to minimize heat transmission to the interior.  Be sure to seal the chassis to the body under the dash, and under the door, where the front vents allow air to flow through the body to the rear- this can be a substantial source of noise and heat.

The rear bulkhead is a tremendous source of both heat and noise.  Adding Lava Shield or some other heat rejection material to the engine side is the beginning, but the edges in particular need to sealed where the bulkhead (almost) meets the body.  On the passenger side, a layer of Dynamat or equivalent, followed by as much insulation as you can stuff behind the FG panel will help as well.

If you mount a coolant tank to the bulkhead, be sure to insulate the tank with something like Technofibra, and use spacers to push the tank away from the bulkhead so air can flow between the tank and the bulkhead.  This will go a long way to making that area cooler.

Another place to add insulation is the front of the footbox, especially if you don't have a plan to direct hot radiator air from this area.  Eventually, the aluminum chassis gets hot when subjected to 200 degree air from the radiator.  More insulation on the inside of the cockpit under carpet will help here as well.