re: I'm going to be building a farmhouse table soon. Any tips?

If you use construction lumber (pine), pay much more attention to contraction rather than expansion. Once it is inside your house, the table top will shrink width wise. If it ever does expand, ever again, it will just go back to the size it was when you built it.
So, your breadboards will always be a little longer than your table is wide. 1/8 to a 1/4 inch on each side.
You may get less shrinkage with the kiln dried, white wood, 2x6s, but those things are like powder to work with.