my 2cents on shorty pumps.
I was working the night (early 1990s) when my old school LT had six 870 barrels, masking tape and a dremel/cut off wheel.
They later became my problem with bead sights that would not stay on or set at 11:30.
One thing I notice during quals with these shorties, hard racking of the action and the muzzle very close to some hands. We did not use magazine extensions on shorties.
We did use the sling plates that had the ball detent (between the barrel lower ring and mag cap) which also served as a hand block to minimize an overzealous hand from slipping off the forend and going forward of the muzzle.
good luck with your project