Had at it again tonight, managing to get the second slab fitted to the tenons. As with the first slab, there are some gaps around the sides of my tenons. It's not perfect. But, it is solid. The slabs aren't going anywhere. I'm still debating about lag screwing them down to keep them in place, just in case they dry and shrink at all.
It took a lot of effort, but I'm glad these are finally fitted to the base. But, I still have plenty to do.
The legs aren't perfectly flush or parallel with the edges of the slabs, so I need to get that done. I also need to plane the top slabs into a single flat plane for a continuous work surface. And I need to plane the ends of the tenons flush with the top of the bench. After all of that, I'll add the casters and vise to finish things off.
Some pics from tonight's effort: