Here is the bench assembled. By far the most challenging part of this project was surfacing the slab; the rest was pretty straightforward, basically just 6 mortise and tenon joints.
I hadn't really thought about how to attach the slab to the base, so what I came up with was a 1' square piece pegged to the legs and attached to the slab with screws in elongated holes:
Seems to get the job done while looking good.
After 3 coats of satin Arm-R-Seal the bench took its place at the end of the table.
This was my wife's idea and I was skeptical, but it is much more comfortable to sit on it at the table than I expected. I find myself choosing it over the chairs.
She wants me to make three more and not have any chairs at the table. To do that, I would have to buy and season another slab, so I am going to try sitting at this bench for awhile before I go along. I am thinking that some guests might not be comfortable without back support so it might be better to make another one for the other end of the table and have four chairs in the middle.