According to page 477 in "Bonded Bomber" by Smith and Rogers in the SAE Transactions periodical from 1962:
"These areas are made of brazed stainless steel sandwich because of the heat of the engines"
https://www.jstor.org/stable/44469505?seq=1#page_scan_tab_contents
The Hustler also has some fiberglass, including the radome and I think the fairings on the top of the wing over the landing gear bays.
I would have loved to see one fly.