I had a friend look at the Balestrand book and there wasn't a Nels Olson on Bøyum but there was a Nels on the nearby farm Viken/Viki. [spellings of farms were not constant either]

This Nels on Viken emigrated the same year as your Nels - the date - 17 April 1870. You will also find that the dates are often off a little. Family histories are rarely 100% correct. His parents names were Ola Andersson Lidal and Marta Ingebriktsdtr. Hatlestad and his birth date was 19 May 1841. Where'd you get his birth date?

The US censuses birth years and ages don't agree with your birth year for Nels of 1845.

So if you do more research in the US and you find his parents' names as Ola/Ole and Marta, then we may be on to something.

There were a number of Bøyum households on this farm[large farm] so not everyone who is a Bøyum is necessarily related to you. Bøyum, as far as I have seen, is only found in Balestrand - an uncommon farm name. Emigrants often say they were from Bergen because it is the nearby and it was the port they emigrated from Norway.

Are you aware of any of his siblings' names? Ingeborg, Mari, Ingebrikt sound familiar?

His oldest daughter was Marta? hmmm Just like Mom.

Too bad the Minnehaha Norwegians book doesn't say much about him.