What reason did the american party give in the 1840s to encourage immigration reform.

The American Party, at its height in the 1850s, was also known as the Know-Nothing Party.  That should tell you something about its stance.  They didn't want any scholarly debate about the benefits of immigration or the role of immigration in America's history.  They argued that those not born in America would only come into the country and take away jobs and rights and power from native-born Americans.  We refer to groups like the Know-Nothing Party as "nativist" groups because they're all about insulating native-born citizens against any perceived threat from newcomers.

The American Party particularly targeted Irish Catholic immigrants who were fleeing Ireland's great famine of the late 1840s.  

If these sorts of sentiments sound familiar to you from talk about immigration that you hear today, well, that's because it does sound familiar.  It's not Irish or Roman Catholic immigrants that are the target of nativist fears today, but immigrants and refugees (often of non-Christian religions) who are coming to the United States from all over the world in search of new opportunity.