My grandfather left Plock in the early 1900's (Szafranski)and also settled in Pennsylvania. I was able to get good information by email from the Plock archives and in March my father and I traveled to Poland and visited the archive in Plock. They were extremely helpful and patient. Have you tried doing a search of their records on your own? http://www.archiwum.plock.com/acts_en_ea.php
I was able to find good preliminary information prior to sending the archivist an email. Hope this helps.