I'm building an FFR roadster and using a Ford 302 with Ford Racing shorty headers has allowed me to not only enlarge the footboxes but 'Super Insulate" them as well.
Inside is the enlarged aluminum footbox. Then, outside, I've added a layer of 10 mm of Pyrogel. Then an outer layer of .050" mirrored stainless steel sandwiched together with stainless button head bolts.
First, the Pyrogel is probably the best insulation you can buy. Cost me $150 and I could almost do two cars including the firewall. It comes in 5 and 10 mm sheets.
Second, stainless steel dissipates heat the slowest of the metals. In other words, of all the metals it could somewhat be said it acts as an insulator.
Third, the mirrored surface will reflect radiant heat.
I got the idea about Pyrogel from Kirkham's Billet build Cobra.
Picture of Kirkham's footbox with Pyrogel
Pyrogel
Here is a picture of my finished passenger footbox and firewall
This shows the firewall with the Pyrogel between a layer of .090" of aluminum and the .050" mirrored stainless steel on the engine side.
I realize this went beyond answering you question but I would try to use the mirrored stainless steel between the footbox and header. If you could then possibly use the 5 mm of Pyrogel or maintain an air space between the stainless using washers or nuts as standoffs.
If you have room inside the footbox you could cut a hole where the heat is getting to and rivet a sandwich, such as mine, of the Pyrogel from the engine side of the footbox extending into the footbox.
Or you could push a small piece of Pyrogel in that small space letting it touch the header and footbox and see how that works. Possibly rivet it in place. As I'm thinking about this the header would probably wear through though.
If you are interested, I can send you, let's say a 6" x 6" sheet of the Pyrogel to work with if you choose to go that way. PM me with your address. Be patient as I'm not on here every day.
Two last comments about Pyrogel.
It
does not take up water.
It is a bit dusty and feels real funky working with it. I use Nitrile gloves and it easily washes off tools with Fantastic or plain water.
George