Converting a 20" 590 to 18.5 inch would be very difficult since the attachment point for the barrel is at the end of the 20" barrel. Plus you bayonet would stick out to far. If you want to save the 1.5" just get a 500. Sell your 590 or get an additional shotgun. I went with both.
A Knoxx Spec-Ops stock, a sling, and maybe some rifle sights are about all I would add to a HD style shotgun but do whatever you would like with yours.