I’d say that barring any territorial changes, one could achieve a more Catholic US simply by not having the 1920 Immigration Laws. By that point, with the exception of Germany (for obvious reasons) nearly all the Protestant sources for immigration were being tapped out due to improving standards of living, but there were suddenly large numbers of Catholics fleeing the chaos of the dissolution of Austria-Hungary, the Irish War of Independence and Civil War, the formation of Poland and fighting off Lenin’s hordes, etc. Had they all been allowed to come to the US, it would have definitely made the US more Catholic (especially the Northeast... somehow...)
I think you are more likely to accomplish this if you didn't have the huge wave of immigration in the late 19th C., early 20th C. the problem is, though, that those waves of immigration were already largely catholic. So I don't think that backing off on that immigration for more immigration later is an exchange that helps any.