Yes, furling mast system.To confirm the exact length for your 420, attach a tag line to the end and pull it through, remove the old line, measure, purchase new line and install by reversing the process with the tag line.
That is what I finally did. replaced with no issues. BTW 70ft is the correct length.You can calculate the total length of the existing line by measuring the various sections and adding them up. For example, measure from the eye at the end of the line to the clew of the sail, and from the clew back down to the outhaul car, and from the car out to the end of the boom, and from the end of the boom to the mast, etc. all the way back to the cockpit. If you're nervous about your calculation, add a few feet as a safety margin.
I used this technique on all of the running rigging that I recently replaced and it worked out fine.