I'm also pretty sure you can swap out top ends on the /5 and /6's. Check Boxerworks.com to confirm, and for other questions, if you haven't used that newsgroup. It's very informative, and the folks there are almost as nice as the folks here.
I have the /7 liter bike and love it. I remember enjoying the feeling of the heavier flywheel you guys have. Although it caused some lateral torque on accelerating, especially in first on the four speed /5 I had, it had gobs of torque and felt great!
I hope you guys have a great trip. If you're coming through the new england area, let me know. I'll ride a bit with you, if I can make my schedule coincide.