Super helpful. Thanks again, Carlos.
So I went out and started looking at the situation, pondering, drilling and cutting.
The fundamental differences between your drawings and what we're confronting are two:
First, the tube I set over the jack points in only slightly larger than they are. There's only maybe 1/8" all around. I wanted them snug but not too snug so I could maybe put a cutting of heater hose over them to keep the chafing down.
Second, the angle of all this is tight and so it the up/down clearance. The top of the main box tube would bisect the little ball on the bottom of the jack point. So the "slot" would have to extend down onto that.
Having said that, I think it will actually work.
I took an hour and made the slotted "caps" and fit them to the jack points.
Since it looks like it will be possible to angle the brackets up from behind, the way forward now is to remake them in cardboard and fiddle with the height of the round tubing until it's just even with the thin part of the jackpoint when the part is installed.
Those tubes are going to only be about 3/4" off the deck when this is done, but they may need to have a rake front to back.
Once the heights are right I'll fix the slotted washers to them, test fit it a few times, then weld and grind it all down neat.
When the washers are collared on the jack points they can't move up or down, so the clamping part will not be necessary.