The problem with the stock downhaul system is that it's not designed to be adjusted from the hulls or out on the wire. The clutch is mounted to the mast and you must be at the base of the mast to change downhaul settings. I would advise at the minimum, getting the Harken 392 Bullet block and riveting it to the mast:
http://www.harkenstore.com/uniface.urd/scpdinw1.showProd?409Z56LC3RA3M You don't have to purchase the entire downhaul kit (or the 392 direct from Harken) and this single component can be found fairly inexpensive for under $100 on some websites. It allows you to use a longer downhaul line and adjust it from the hulls or out on the wire. The blocks allow you to make a downhaul adjustment and then lock it down like your jib blocks.
As it is, you would have a 4:1 purchase. If you have double bullet blocks on your sail and would like to make it 8:1, you would have to buy the small Bullet cheek blocks and rivet them to the mast over the steel plate of the 392. It all makes life so much easier to have downhaul controls on each side of the boat at the hulls.