You'd be much better of to just buy a different barrel.
The 870 has a large number of types of barrels with sight and rib options.
By the time you pay a custom gunsmith to do the difficult job of installing a rib, never mind finding a gunsmith who can do it right, you could easily buy any type of barrel you want and then some.
Check online sellers like Midway for new Remington barrels, check Remington themselves direct, or see your local gun dealers to order.
Many local gun shops and pawn shops will have new and used 870 barrels.