Funny; Most of the people who have inherited Clark's or purchased them off of Craig's weren't even born when Grubby was in his hey day and in some cases not born until after Clark closed. So Charlie's statement about being half the age of the blank becomes more relevant with every passing day. PS-- my thought is; If you've got 'em, shape 'em.
My first recommendation is to stick with the 7'6"x18.5" as is, and do your best effort to make a BK Sunset semi-gun single fin. If you are set on a Sting, then get some rail cuttings scrap PU foam, clean up the edges, clean up the edges of the blank, and bond them to the blank with Super 77 spray contact cement. Don't gum it up, else you won't be able to shape through the bond. A light dusting on both surfaces shoud suffice. Then get on with your project. I've given up on Stings, and a little over a year ago I did a re-shape to convert my last one to a winged swallow.