RE: SABOT SLUG SETUP ADVICE??
I would recommend trying a few brands of sabot slugs, starting with the "cheaper" ones and working up to the more expensive ones, until you find one that the gun shoots well enough for your hunting. Your maximum effectiverange will be somewhere between 100 and 150 yds, depending on your results.
I shoot a Marlin 512 slugmaster, and it does well with the cheaper Win Super X 2.75 or 3 in sabot slugs at around 1400 fps. It did not like Hornady H2K sabots at all. Recently the Win. slugs went up to about $12 a box, so I switched to Brenneke KO Sabots ($8 a box when I bought them) at around 1500 fps and they shoot well too. I would take a 100 yd shot if needed, but most of my shots are within 75 yds.
As far as adding to the gun, I don't see that you need anything else. I know a guy that added a pad (Bear Creek I think) to the regular 870 stock, so he had something to rest his cheek on. I don't know the configuration of the stock you bought, so maybe it already has enough of a comb for you.