I just checked the remington sight and it looks like the only lefty 870 is the express with a 3" chamber and 28" barrel. I'm lefty too and I have 3 870s. A 20 ga youth camo I use for turkey, a 20 ga express 28" for grouse, and a 12 ga Express Super mag, 28" barrel 3 1/2" chamber for waterfowl.
I have no problems using these right handed shotguns except once in a while I get powder blowback when shooting into a stiff wind and cycling the action.