I went through this with one of my other cars a couple of years ago. I was eventually talked out of wrapping the headers and ended up getting them coated with a 'heat proof' material. The coating was done by a local Australian company called Jet-Hot (
www.jet-hot.com.au) and I am very pleased with the result. I would think that similar products are available internationally.
The under bonnet and on bonnet temperatures are significantly cooler. The two temperature photos were taken before and after treatment and after I'd been driving around the neighbourhood. One probe was about 10cm away from the headers and the other on the outside of the bonnet above the exhaust side of the engine. I did this out of interest only and have no affiliation with the company who did the coating.
The other two photos are just the before and after of my headers.
I've not done this to a 500 yet as mine is still in bits
As to the materials making up the 500 standard motor, I believe they are -
Sump - mild steel
Crankcase - alloy
Cylinder barrels - cast iron
Head - alloy
Rocker cover - mild steel
The engine bays of these cars were originally fitted with a heat shield bolted to the RHS of the engine compartment and offering partial shielding of the exhaust. I don't know how effective it was, or whether it can be made more efficient.
Check the airflow around the motor - particularly underneath around the sump as this area is prone to damage, road dirt, grime and general greasy crap that will block the flow of air. A significant amount of cooling takes place here. My sump ventilation was virtually occluded and is now panel beaten straight again and clean
Also make sure your thermostat is working and the recirculating vent is opening/closing properly.
Hope this is of some help,
Chris