After a little research I determined it was Briggs Cunningham driving the No. 31 Porsche. Cunningham was the captain of the winning Americas Cup sailing team in 1958.
This car finished 7th in the USRRC GT race pictured (May, 1965) behind the Ken Miles Cobra (shown in the previous picture) which finished first, and Jim Hall driving a Chaparral that finished second. Here's a blurry image of the Chaparral -