Keep the length, as has been said. Go with the Harken track and traveler car:

http://store.catsailor.com/tek9.asp?pg=products&specific=jndmfrpok0

(that should be one line) In any case, right here in Cat Sailor City, you can go to the store, do a search for 'traveler track' and get a page with a listing of various tracks. Pick the best for your specifications. If in doubt, go bigger. Do the same for 'traveler car'.

Good luck Connection: close
with it. At least there are lots of convenient holes drilled in your beam so you can undo the screws... imagine trying to reach something to the centre of the crossbar, whoo hoo!