I identified the race and some drivers at Road America:

Race 7   (June 24th, 1956)

#16    Bill Lloyd                       Maserati 300 S
#60    Briggs Cunningham      Jaguar Type D
#66    Phil Stewart                   Maserati 300 S 
#111  Carroll Shelby               Ferrari 121 LM  (s/n 0558LM)

(8-2-09)  From Steve Funk:

"That’s my grandpa Lou Brero Sr. in the zebra-striped D-Jag in third; I believe he was running #152 for this race.  Behind him, I believe, is
Ernie Erickson in #12 D-Jag.  Finishing order:  Shelby first , Brero second,
and Erickson third overall."