Uncle Mike's used to make a drop-in lefty safety. I have one in my 870, but tought to find one these days.
It's shaped differently than the stock safety and when installed does not rotate at all like a righty safety does. It also came with a plunger pin to replace the ball bearing detent. The exact same parts are also used to convert the 760/7600 rifles. If you post a WTB, someone might have one they'd part with.
UM doesn't make them anymore and shops generally won't sell anymore as they are all afraid of being sued if you install it and then shoot someone with a gun that "you thought was on safe". Funny how lawyers have managed to negate the "don't point a loaded gun at ANYONE, safety or not" common sense practice.
Also, Remmy makes a left version of the 870 now (that most aftermarket parts don;t fit on - grr) so the makers don't feel the need to pursue conversion options.
As a lefty, I like the righty 870 with lefty safety best as the right handed slide release is perfectly located for a lefty - go figure.