Assume you are
talking about buying a headsail, not the furling hardware. If so, the C34 furling headsail should work on your C36, assuming the luff tape is the right size for your furler. As you indicated, the luff will be somewhat shorter than your current system can accommodate so that (1) you will lose some potential sail area and performance*; and (2) you may need to use a pendant at the head of the sail in order to avoid the potential for halyard wrap. *Depending on the overall square footage of the sail and its other dimensions, that is, it may have a longer foot than your current sails, performance may not be negatively impacted and could possibly be improved, although this is unlikely.