Duuuude... you're most the way there! The top looks reasonably flat and the bottom just DOESN'T matter. Like a ceiling, nobody will ever look at it closely. But even if they do, make it equally triumphant. You don't want to attach a cleat for the legs across the grain anyway, that would just tend to pull the two halves apart when the joined slabs expand and contract with the weather.

So, the undercarriage only needs to touch the top in three or four places. Easiest would be to attach (glue and pegs or screws) four blocks to the underside of the table, build the leg assembly, and then scribe the tops of the leg assembly to the blocks. You can do this by laying the top on the floor upside down, place the leg assembly next to the blocks, shim the leg assembly level, and then mark the difference between the legs and the blocks on the legs.

When you trim the legs to these different heights they will all contact the top. or the blocks depending on your attachment scheme at the same time with the feet now all level.

This is easier to draw than it is to describe. You might have to scribe the blocks to the underside of the table, shims underneath, hot glue or double sided tape to attach them temporarily, then transfer the contour of the underside to the blocks with a protractor held at 90 degrees to the top, pointer end slid along the underside while the pencil transfers the contour to the block, then carve and whittle to that line.

You could also thru tenon the top ends of leg assembly to the top, glued and wedged. Hope some of this inspires a great idea, the mark of a craftsman is the recovery from his INEVITABLE mistakes;)