If the block in the photo is just stuck in there for show, pull it out cut a small 45 bevel on the edges of the filler blocks so you can get it down into the walnut leg if the glue is in the way.
If that block is glued into the leg on the cheeks it should be plenty strong to hold the hanger bolt for the leg.
In some of this thread people are talking about the table expanding and contracting in a bread board end. I can't imagine it changing by a 1/4" during the course of a year unless you live in a climate subject to humidity extremes and no heat or AC. I have an oak kitchen table with breadboard ends I built 8 years ago and in the summer it grows not quite a 1/32 past the ends, right now its shrunken not quite a 1/32 in the ends.
When you make the breadboard end piece you can router in the dado or cut it in with a saw or dado head and chop the mortices square with a chisel.
You also don't want to run a full dado, maybe two so there is some wood at the center holding the top and bottom of the breadboard together. The end gets pinned on to the tennon.
Firemedic posted a tutorial on breadbord ends-
Look at the photos esp. where they show slotted grrove for the outer pins. I didn't run the dado all the way through like he did, but his table is a little more rustic than the one I built.
Cut haunches on the end or the tennon and don't run the dado all the way through so you don't see the tennon. Pin the tennon onto the mortice and don't glue it of you can just glue a small portion in the center to let the table field expand and contract
The clips from Rockler would work OK to attach the top, I used some aluminum Z in a similar fashion.
My kitchen table-