I used 100 year old wood, so I had to be very careful and judicious about sanding. Don't plane reclaimed wood, at least on any surface you intend to keep rustic. I lightly sanded and cleaned up the grains and ran the bottom side through my planer. The hardest part was getting all of the boards straight with out losing all of the really cool character. They were warped, cupped, cracked, chipped, rotting, you name it.
Typically I would just edge glue and clamp a table top. But, this one is unique. The individual thicknesses vary all the way across, so I did miter in a groove and hammer in some 1/8" hardboard. It kept the table top as perfectly uneven as I wanted it to be.
The legs I attached to the ribs that I mounted on the bottom. I set up a jig and ran a router to make an even and level surface to glue and screw them to. The routed area is the exact shape of the ribs, so they are inset about 1/4" on average. The legs are bolted to the ribs in 4 locations. It ended up being more stable than I anticipated so I canned my cross beam I had started to build.
Does that all make sense?