This got me thinking. Somewhere in the past, I remember something about root diameters being different for fine threads and coarse.
So I ran a die down the Dr. Diff bolt (linked to in post #13), which was 7/16-20 as received. I figured the worst that could happen is that the threads would be ruined, and I would proceed with my weld-and-turn plan.
The first picture shows after I ran the die. It worked! Second shows the installed bolt.
I know myself well enough to know that I can, on occasion, overthink sumpthin. So, I un-thinked this one! (Insert back-up beeper sound.)
Thanks for the ideas, they're what got me there! Plus, if someone searches this with the same issue, here it is.
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