yes the rivets line up with the old ones. the refinishing is just because of this: if you look at the side of your 870 opposite from the ejection port, you can make out two faint 'dots' (usually slightly darker) - these are the heads of the rivets that hold your ejector/ejector spring. theyve been finished flush with your receiver and blued along with the gun.
now if you look inside the port, you can see one rivet holding the ejector spring. there is another one underneath the spring on the other side as well. you can buy a $40 cutter from Brownells that will let you trim the existing rivets so that they can be punched out, and hopefully they have enough meat on them so that you can re-peen the existing rivets.
otherwise if you
cant re-use the old rivets, when you install the
new rivets their heads will
slightly protrude from the side of your receiver. when a gunsmith does the job hell finish these smooth with the receiver and blue the entire thing so theyre invisible, but this drives the price up since youre also paying for receiver refinishing. if you are planning on having your 870 parkerized/gunkoted/teflon coated anyway then that would be a great time to do the 3" upgrade.
if you are mechanically inclined at all, then its a simple job to do even if you dont have the specialized tools. otherwise let your gunsmith do it: they usually only charge about $30 to replace them if you have the parts, and with luck they will be able to re-peen your existing rivets and you wont need a refinishing. if this is just a hunting gun and you dont care that youll have two small 'buttons' sticking out a millimeter from the side of your receiver then just dab some cold blue on it and be done with it. if its a 'tactical' style build then your sidesaddle will cover it anyway.
as for the barrel:
i honestly believe that any money spent on the old 2 3/4", plain wingmaster barrels is money wasted. i have seen the 28" vent-rib, 3" chamber Express barrels for as low as $120 on the EE, including one choke. this, IMO, would be a far better investment for your money. yes, its Express finish compared to gloss blued Wingmaster finish, but if you are using this for hunting then it doesnt matter.
sell your old barrel for ~$70+shipping on the EE, someone will buy it to cut it down. put that money towards a new barrel.
one rivet (other one is under the end of the spring and visible once the spring is removed):