Your title sounds dirty, LOL.
This is what you want.Due You just didn't know you wanted it.
Use a propane torch to solder these nuts into the tube. Use flux.
Yeah, the seam might potentially cause problems with tapping. Also might interfere if you go with the tube end nuts I suggested. The main reason is the weld seam is usually harder than the rest of the pipe due to the rapid thermal cycle.
Best option is to use a carbide burr bit in a die grinder or chuck in hand drill to file down the weld. Could also use a round file if you like a crap load of elbow grease.
Option #2, hammer forge the tube into a square shape. Then heat it red hot with a torch to stress relieve it.
Then grip the tube in a bench vice and use these screws: https://www.mcmaster.com/94077A417/
These screws swage threads into the material instead of cutting them. Apply a generous amount of oil or grease to the screws before running them inside. Run the screws back and forth a few times.
It may take some experimenting to find the amount of squaring thats ideal for get the head of the screws into the tube.