I had a Beretta 687EELL 20 ga. and bought a 28 ga. barrel set for it from Joel Etchen. The 20 ga. fore-end will fit but there will be a small but noticable gap between the wood and barrels. Joel Etchen sold me a fore-end with the barrels - I vaguely recall the total price was $1200, which included having a local gunsmith do the fitting and refinish the fore-end to match the stock finish.
You might want to call Joel Etchen to compare prices. Coles is another option both to buy the barrels and to do the fitting - they charge $150 to fit barrels bought from them, $200 for barrels bought elsewhere.
