Gustav Nilsen Reistad of Drammen, b~1855?

My great grandfather died when my grandmother was only 12, so I have precious little oral information on him. His Marriage license, dated 4th of May, 1881 in Cook county Illinois lists his father's name as Nils Christman or Christensen and his mother as Maren Larsdotter. He was to be 26 or 27 on his next birthday, so his birthdate should be between May 5, 1854 and May 4, 1856. The place of birth is listed as Drammen, Norway. I presume he took the name Reistad when he emigrated. I have Ancestry.com, so started looking in the Norwegian records, but have not found his birth to these parents. I am willing to search records, but would appreciate local knowledge of which parishes are located nearest to Reistad. A witness to the marriage is Martin Nielsen Reistad, who my father has always assumed was a brother. Joliet Illinois directories indicate that Gustav and Martin were machinists and salesmen. When Gustav became an American citizen, he noted that he was of Norwegian AND Swedish heritage. This makes me wonder if he was born of Swedish parents who happened to live in Drammen as he was born, or if they moved to Sweden and he grew up there. Were there any common emigration patterns in the 1860's that might hint at where he grew up? I have searched some of the ships records as well in the years before the marriage, and have found similar Gustav's but no Martin's. Is there a Norwegian name which is commonly americanized as Martin? Any help you can provide or suggestions for further research would be deeply appreciated.