You will need to research the current materials, most of my work was in the late 80s. I did some fabrication at that time. Since then only repairs and touch-ups.
I used blue polystyrene, mainly because I was doing a lot of hot-wiring.
From my catalog, it appears Last-a-Foam works well also.
You will want one of their catalogs, very instructional.
418 miles on 19 gallons of gas. Can't beat the BBD !