Here a pic of what my 8:1 downhaul looked like before I started mucking with it. Not exactly the same blocks, but the layout is the same. The system in this pic (out of the Murrays catalog) uses one back plate for all the blocks at the bottom, mine has two separate ronstan swivels and the bottom block simply screwed to the mast base.

Keep in mind that somewhere in between the top and the bottom the boom connects to the mast.

I added two lines from the boom gooseneck, to a shackle with two ears attached to the sail tack and down to each of the blocks.