Modified or not I think the best way to go is with a Sound deadner and some kind of heat barrier.

I didn't have much luck with the Frost king stuff my self. It didn't stick very well for me. Others have had great success.

The firewall is something you only want to do once IMO. I would spend the money on some Damplifier Pro or Dynamat first. Your factory firewall pad may still be the best bet for a heat control second. There are other good products for heat but the factory pad is a quick and easy install.

If you going to spend any driving time in the car other then a 1/4 mile at a time it money well spent.