Cabela's occasionally has the Baker Cavalry shotgun for sale and the longer barrels will fit on the stock. There you'll have the shorter barrels for "close encounters" and the longer barrel for the pheasant hunt.
Could also get the Howdah, which also goes on sale on occasion for much less than the Baker does, but it doesn't have a buttstock. Pedersoli does make a detachable buttstock for the Howdah pistols, but they're $250.
At least going that route you could have the 20 gauge pistol, the 20 gauge short barrel shotgun, and then the long 12 gauge barrel all on one platform for the same price as just one complete Pedersoli shotgun.
I'd put the extra barrels on gunbroker or ebay. People like me who wouldn't mind having all the different barrel lengths and can score a deal on one of the 29" barrels would be happy.