a question: if I use the two sheaves to the mast where I pass the halyard which currently passes over the right side sheave?
You can install up to three sheaves on the 17's mast base, which is what you'll need to do. If I recall, the original 17's came stock with only two sheaves. The halyard passes through the center sheave, the downhaul uses the two outer sheaves.
Marcusnans you might want to look at some catalogs:
Hobie Parts, page 25 (on my version) or Murray's page 26.
Neither Hobie nor Murrays make an upgraded downhaul system for the Hobie 17, so you're forced to "cobble" parts together. What's wrong with using your own brain to solve the problem? In my experience, these pre-packaged systems are almost always over priced for what they are and are usually far from perfect anyway. In any case, he's already go the exit blocks, so all he really needs is a few more pullyes, a longer line, and he's there.