If it weren't for the need to box it up, I would highly recommend the Big Dummy. A one-piece frame is going to be less flexy than two pieces bolted together. Also the Dummy is a very stout bike. Stoutness of an Xtracycle depened on the donor frame.

However, since you need to box the bike, I'm going to go a step further, and say you shouldn't be riding a longtail. I'd recommend a good sturdy touring bike. Don't underestimate what can be carried on an LHT, Fargo, or other bike built for loaded touring.