Your best bet IMHO is to just buy or trade for a 20" smoothbore 870 barrel that has rifle sights installed from the factory. I see them on Gunbroker for $100 and up... should be one at a FFL shop around where you are, I'd think.
Shop before you chop... bubba'ed shotgun barrels are worth almost nothing.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.