immigration

My great-grandfather Amund Johansen Slaastuen, his wife Marit Hansdatter together with 4 children immigrated to America about 1880. I am trying to pin down the exact date. They were from the Kvikne parish in Fron, Oppland. They are on the 1875 census from there. They were in the USA by 1883 as the older daughter was married then. I have checked the records of the Kvikne parish in hopes the date of their departure from the church was recorded but no luck. Any suggestions as to where in Norway I might find more information? Marlon Henvit Reno, Nevada USA