Some experience from Hurricane #6, which has a 427 FE and now has 6k miles in South Texas heat and humidity. More important than any hear barrier is the flanged headers and side pipes. If you have the slip-fits, you need to flange them before doing anything else. If possible have at least the headers ceramic coating as its does make a huge difference.
With regard to the footboxes, I doubt an air gap would provide any measurable temp difference. What will significantly affect foot box cabin temps is using some sort heat barrier sheeting. I bought a few pieces from Pegasus Racing and used them behind the footbox sheet metal, which is then riveted in place using 1/8 rivets 3" apart. If you buy the right length rivets they will pull and hold just fine no backing required.
I also used the same barrier inside on the front and side footbox walls under the carpet. The interior floor area was brutally hot until I flanged the exhaust but now its fine. Short of Space Shuttle tiles there is nothing you can do to completely negate the heat. But the barrier material is far more important than any air gap in my opinion.
Here's how I did mine and I have not had one single rivet come loose:
