I would use dominoes on planks that big, and cross members on the bottom, and bread board ends, Man where in the world did you come across those planks? My neighbor has an Alaskan saw mill, and is going to make a table out of some lodge pole pine. One is from my property. You can plane the boards flat, with a long hand plane, but it needs to be very sharp. I think it would be easier to do the top, after the planks are glued together. You need to check the planks after each shaving, with a straight edge, side ways, and length ways, and that way, you will be able to see the humps and bumps that need to be taken off. Good luck. Man I would love to find some planks like that!
Wayne