On my nacra 6.0, we run two sets of spin blocks, one on the rear beam and a second set further forward. I have one set of the harken ratchematics and a second set of manual on/off blocks that we turn off in light air. Remember that with ratchet blocks, the angle the line comes out of the block matters cause of the numberr of 'teeth' inside the block that have a grab on the line. In my particular set-up my crew can hold the line fully heated up with both ratchets on with two fingers with the NE class spin @ 350 sq ft. (pulling in the sail is a lot more work though).
Another option to consider is using a 2:1 spin sheet often called a 'doubler'. In this case a set of blocks is tied in the sail @ the clew that the spin sheet is run though and a stop knot tied on the other side. When you need the extra power, the crew reaches up the the stop knot and pulls it back to a cam cleat on the rear beam. When its time to jibe, release the sheet from the cam cleat and you are back to a 1:1 sheet so you don't need ten thousand feet of line.
hope this helps...

Chris