Uncle Sam,
"A" for the day!
The original footbox in both the 289's and 427's were in deed fiberglass. They used a hair type glass construction with a gel coat off white finish. The exterior finish was what you saw on the slabside. The right FB was where you will find the V.I.N. tag. Note on some there is a spaced ally sheet, help with heat..... Don't think this heat shield was a std option from SAI.
Any pics of the Slab you can share?