OK, this morning I welded it, and yes, the allthread was tight but it came right
out with Vise-Grips and then I was indeed able to chase the threads with a tap
(they only bound right around the weld area, as though they'd somehow shrunk
right there) and then the threaded piece moves smoothly throughout. Cool. I
wound up doing something different than I'd said. I faced off the coupler nut
and installed it on the allthread in place, but not jammed. There was actually a
very small range of radial location where the allthread didn't rock slightly,
and the threads didn't bind at all. I welded it right there without clamping,
then like I said, chased the threads. Bingo. - GWE

Grant Erwin wrote:
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.