Well, I put on my Big Boy pants and headed out to the shop to teach myself how to cut square threads. I'll be rebarreling a Remington Hepburn in 38-55 and never cut a square thread! First I measured everything and wrote it all down. Then I ground a cutter to .049" square, set up a piece of aluminum stock in the lathe, set the lead screw pitch to 12 and started the cut. The depth of the cut turned out to be .0395" for a nice tight fit. All measurements were taken off the old barrel and receiver. Really wasn't difficult at all. Now off to Green Mountain Barrels to order a .375 barrel to be chambered in 38-55....
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Roy,
I cut quite a few square threads, Buttress, Acme, modified Acme and V but that's beside the point.  A trick I stick to when I grind tooling for these, only grind enough to win, nothing extra.  Most tend to grind a length 5 or 8 times as long as they need, if you're threading up to a shoulder, grind the tool on the leading side with just enough clearance to stay out of the shoulder, the other side far enough to clear the thread.  Hope that make sense, rigidity is your friend with a large flat worth of tool contact.  One last thing, if your lathe permits, turn your compound to 90°, it makes the little width tweaks easier and picking up a lost thread better too.
Square threads really aren't that bad, your only contact is the front, don't run a positive or build in a chip breaker, a neutral or flat rake is better on the tool's top.  The front clearance can really be kept at a minimum, just enough not to rub - 5°
Attached a couple pics of remnant tooling from jobs with square threads... Both were from fairly large size jobs...
My 2¢
GT

one divided by Threads per inch divided by 2 will give the thickness of the cutter, add a few thou for clearance. Just did one yesterday. 12 threads per inch shows a cutter that is .0416", add .003" for .045" works fine. Depending on how strong you lathe is a depth of cut about .010" per pass will work fine.

Back in the day...🤨  Correct me if I'm wrong but you don't need any Relief on the Trailing Edge.   That helps, one less fussy area.  I would do 2 things, Dress my Grinder so the Face was true and then grind within .002 of size and Finish to size on a bench stone.

Reading the replies, Hats off to all!  😎