I cut and pasted this out of my GP book to help you out. Just change dip switch S8.
FREE PLAY OPTION (MPU-1 or MPU-2)
The game has provision for allowing free play. When the free play switch is on, credits are decremented normally until 0 credits, then pressing the credit button puts 99 credits on the game and they continue to be decremented.
FREE PLAY S8
ON ON
OFF OFF
Here's rules and scoring for SS on the site, too.
http://www.gameplanpinball.com/machine-sharpshooter-rulesofplay.shtml