Update- After repeated delays caused by waiting on shim plates, rain, work and grand kids, I finally got the grapple slop fixed.
I ended up using the 3/16" plates, provided by the manufacturer, welded on top of the grapple's bottom mounting tabs to fill in the gap. These made the grapple fit snug to the loader's attach plate however, the locking pins still wasn't making good solid contact with the plate when locked down. I first tried tacking a piece of flat 3/8" X 1 1/2" on top of the plates but the pins were dragging on the top of them which made the grapple hard to attach and detach. I didn't like that. I thought flat 1/4" X 1 1/2" wouldn't provide enough contact to the pins. I didn't like that either.
I found some 1/2" square stock in the shop so I welded the correct lengths inside the slots to fill in the space with good solid contact. Now everything locks up tight and the locking pins don't drag when attaching/detaching.
The grapple mounts and dismounts easily, I can back drag and there's no grapple or locking pin movement at all.
I hope to get the welds cleaned up and painted this afternoon. I still want to tidy up the hoses a little and add some sleeves/spiral wrap for protection but there's no rush on that.