The 21"(Nominal) bbl on Frankenstein, my generic 870 parts gun, was done like that and threaded for tubes.While bobbing the bbl can certainly be done safely by someone handy with tools, a better job can be done by a smith with the right jigs,etc. I'ts not an expensive job.
And I've seen those magnets come off on a trap line, so back up the installation with epoxy, black tape, double sided tape, or baling wire.
And a used 870 with short bbl,mag extension, and Hi Viz sights comes mighty close to optimum homeowner's insurance,in my admittedly biased view.