I identified the race and some drivers at Road America:

Race 3   (June 23, 1956)

#5      Jim Kimberly              OSCA MT-4
#6      Ebby Lunken              Ferrari 500 TR  (s/n 0612MDTR)
#7      Ted Boynton               Maserati A6CGS
#30    Bob Goldich                Ferrari 500 Mondial  (s/n 0536MD)
#61    Briggs Cunningham   OSCA MT-4
#75    John Quackenbush    Ferrari 500 TR  (s/n 0634MDTR)
#212  Mike Marshall           Porsche 550 Spyder

(7-29-09)  From author and historian Willem Oosthoek:

"It is Mike Marshall, the Miami Porsche dealer, not Marschall."