Brian, remove the recoil pad with a Phillips head SD, then use a long flat SD to tighten or loosen the stock screw. Overtightening may crack the grip through the wrist, so hand tight is OK.While you have the stock off, coat the threads with beeswax to protect from rust and still keep everything tight when you put it together.
As for vintage 870s, heck yes!! Buy one for each kid you have. Buy one for your wife. Then buy a spare for parts. Might be the best investment in the world.