[Charlton]

Jim. I am by no means an expert on your question, however, I have your same car with a 428 in it. I assembled it in my garage. My exaust pipes are maybe 1" from the foot box. I installed insulation padding between my carpet and fiberglass on the inside and a 1/8 inch heat cloth made from ceramic material on the outside of the footbox.
I also had the pipes ceramic coated and wraped the #4 and #8 pipes with a thicker ceramic blanket. I did all that so my feet wouldn't melt. I never thought about fire.
Also, I sealed all openings from the engine compartment to the passenger compartment.