Hot air manifolds and new firewall

[Coupe35]

WOW, these hot air heaters put out the heat. This is my first and only experience with one and when I installed it, I was not expecting much heat. But on a 30 degree day it will run you out of the car. And the engine temp did not get over 150 degrees. My question is. I would like to install a new firewall insulator, but since they are made out of plastic, I’m wondering if the area around the vent will melt. The vent gets so hot, you cannot touch it. For you guys that have a hot air heater and new firewall what’s been your experience?