Here's the deal. Weld metal shrinks, so where things get welded, pull. I figure
if I run thread through the coupler nut and into a threaded hole, then just weld
up the nut, that the nut will be pulled a few thousandths closer because the
weld will pull it closer and the metal on the end of the coupler will compress a
little.

I noticed a 17/32" drill will just fit through the coupler nut. 17/32" is .531
-- well, I drilled 3/8" then 17/32", and the hole came out .538" not .531" -
I'll have to resharpen that drill bit, but when I turned down a scrap bolt to
..537" for a slip fit it no longer would pass through the coupler nut. So it
would go in the hole in the part, thread on the coupler nut, weld on the coupler
nut, then I'd have neatly plugged my hole. Oops. So I just went ahead and
threaded the hole in the 3/4" part. Now I'm going to put in a piece of
allthread, put on the coupler nut, run it down to where it's just starting to
jam the allthread, and then cobble up a beefy little clamp to pull the coupler
hard against the 3/4" part. This will emulate the pull of the weld metal. If the
allthread moves smoothly without binding at that point, I will weld it up. If
not, I can readjust things until it moves smoothly without binding while clamped
very tightly.

I don't want to have to chase 2" of 5/8-11 thread. Don't think a standard tap
will do the job, and I sure don't want to buy one.

Will weld it up real soon, and post what happened.

Grant

I still think the best bet on this little project is to get a suitable
length and size of all-thread, drill/tap the hole, thread the nut onto
the allthread, then the all-thread into the hole, run the nut down the
all-thread to butt up against the hole like you would a jam-nut, but not
"jam" tight - Just run it down until you've got contact with the base
plate the hole is in, then adjust until the all-thread turns easily and
start welding it down. Once finished, unscrew the all-thread from both
pieces, and I would expect that things will be golden.