As Jon correctly points-out, the loading on the block is going to be maximized. On our J/Boat we have blcoks that are aft for the spinakker (even though it's a 2,000 SF spinakker)-- but it's not a snatch block. I'm not sure you couldn't get buy with the block being farther foward? It depends on the sail, you rig, and whether you are running the chute close.

In fact, when we're running deeper, the chute trims out to the hit the bottom of the boom. We use a twing sometimes -- but in those conditions, we would be fine with the lead being slightly forward of the primary winches.

Having said that, I'd still use a strop in the track if you use a snatch block or similar attachment.