Google is your friend. I would advise you to Google
"Herbert Malm Jr." New Mexico
"Jeff Stephens" New York
Just like that with their names in quotes. That is the best way to try to find someone. Unfortunately, most original or 2nd owners of S351 Mustangs were not real Saleen enthusiasts like you find here on the forums. The reason being these cars were so pricey brand new back then they were mostly purchased by collectors who don't even know how to operate a computer. It's the truth.
The true enthusiasts that frequent these forums usually have to wait 10 years or so for the price of the cars to come down to our level. You never know though. There may be someone here that knew or knows one of those guys.