It all depends on the size of your sail.  CLC used the 1/16" wall tubing for the mark II sailrig, with the 55 sq.ft. sail, but increased the tubing to 1/8" for the mark III rig, with the 70 sq.ft. sail.  The 1-3/8" extention is to be used as a re-inforcing sleeve inside the two, or more, pieces of 1-1/2" tube, where they join to give you the 17' total length.  CLC may have a source for the 1/16" wall tubing.  (If you have to source it yourself, I would look for 6061 aluminum.)  For a discussion of the history of the sailrig design, read John's blog in the Life of Boats called "Kayak + Sail = Fun".  

The 1/8" tubing is heavy!  I have the mark III rig with the 1/8" tubing, mounted on a WoodDuck 12, and I'm still working out the trim issues.  The 1/8" wall mast weighs around 20 lbs, or half the weight of the kayak, and is causing the boat to ride bow-down, slightly.  Next time I go out, I'm going to add weight inside the rear hatch to try and even things out.  Personally, I'd love to be able to get away with 1/16" wall tubing just for the weight issue, but that's not possible.  Too bad the sail won't work on one of my old carbon fibre windsurf masts, because it requires sailtrack for the mark III sail.  That would solve the whole weight problem.   But that's something you might consider, since your sail sleeves over the mast.  The only difference would be the diameter of the mast at deck level, requiring a different diameter hole in the deck and mast foot.  Just a thought.

   Thanks for info Peter. I have read John's blog, and have a much better insight now into the mast setups. I wasn't sure of the strength of the 1/16" wall tubing even using 6061 T6 because of the fatigue properties of aluminum, and apparently the Mark 1 and 2 designs do flex under sail. One thing I have learned over the years though is don't question experience, so I'll follow the plans.