I suggest looking up heat shielding on the Summit website or catalog if you have one. They have a few options of heatshields that are pretty thin but still have a high temp rating. The brands names are Cool It and DEI. I will be using something like this on my 39's firewall. I put a 350 in my 48 Fleetline and just used the cheap roll heatshield from a local hotrod shop and it does not do much to stop the heat.

thirtyniner

39 chevy 1/2 ton
48 chevy fleetline aerosedan