The lug on the barrel extension has been extended by welding. David Parker offered to take on the job of welding up the lug, and he did a great job.
I had asked for an oversize buildup of weld, so I could mill it down. So, here is the results:
1 - First pic shows the weld buildup as received from David.
2 - Second pic shows the top and three sides of the lug squared off. I left the sides and top .010" oversize. I can take it down more later, if desired.
3 - First test fit: The lug is .055" too long. I ended up making six cuts off the front of the lug to get it to where I think it should be.
4 - Barrel extension is now level with the front of the frame. Everything clears; everything functions.
5 - Side view of completed lug modification.
6 - Pic of the takedown lever and barrel extension lug showing how little engagement there is between the two (which was my original request, to see how much should be engaged).
I had estimated it would take .040" to make it flush; it took .050". I'm a happy clam!