I'm in the middle of rebuilding my Harken traveler and have purchased the windward sheeting car conversion kit. I also loaded new torlon bearings in the car. My plan is to start with 4:1 control lines in a continuous loop. The loop will have about 24" extra. I also will run the traveler through an eye outboard port & stbd with jam cleats. This allows the outboard cleats to be used to stow the traveler since Harken recommends something similar when on a mooring or in conditions where wave motion in light air may cause the windward sheeting car to slip.

This gives me two options, both that shorten the traveler control lines if I find I don't like the setup.
1. Shift from 4:1 to 3:1 as a result of bypassing a block and moving the end point termination from the center to the edge.
2. Cut the continuous loop and go single ended on each side.

So I recommend buying sufficient line to run continuous with a couple feet extra. Try it out, then see if you like it. You can always cut the line to shorten it if you don't like the setup.