First of all, you can't fit 28ga bbls to a 20ga frame. If your gun is Italian,i.e, #470xxx, you can not fit the Krupp bbls to the gun at all. Galazam is selling Krupp barrels in 16&28ga. that will fit the VGM (#471xxx) 20&28ga frames respectively.
Let's assume that your gun is the correct type. It will cost you between $1,500 to $3,000 to completely fit and finish the barrels. You can't recoup the cost.
Sell the gun "as is" and ask $6,500. I doubt that you have that into the gun and may be able to get your asking price if it is a Krupp/VGM gun. I value the Italian guns at 15-20% less, but, that's me.