Longhouses are certainly part of the Indian history and culture.  Some, of course, were built by other cultures, but here in the U.S. Indian tribes built them on the East and Western Coatlines, and I think there is one in Estes Park, Colorado.  Some Foundations have come together to preserve these structures, and to help the tribes that still need support for their families.  I would be surprised if they would allow people to get married in there, not because there isn’t room, because we know that they are often 200 feet long.  They may be sacred to the Indians, but they were dwellings for extended family, and so the idea of a couple being their life at a ceremony in one, seems to be a lovely idea.  The only way to find out if it possible to have your ceremony in a longhouse, would be to contact either the Indian Tribe who owns one, or the Estes National Park System may be able to give you information, or a source for information about this subject.