We adjusted the plans so this table top measures 84" x 48". Our chairs were also a little taller than normal, so we raised the legs to make sure the height of the table would be comfortable with the chairs.
I've lost all the math we did, so I'm sorry I can't help you make these adjustments. We made this table a couple years ago.
We used 4x4s for the legs and I'm really glad we did. A previous Ana White table we made, we used thinner legs and the poor thing squeaked like crazy. This table is much sturdier. We also used thicker pieces for the top which also adds to the stability.
A friend helped us plane the table top and fit it all together. It's just pine though, so we have had some warping, but it isn't bad.
We bought a kreg drill to make the assembly smoother. Love that thing!