It's kind of a shade tree method, but how about putting a pair of big C-clamps on either side of the center bolt, remove it and replace with a piece of all thread rod, or better yet a long, Grade 8 bolt with a full depth nut on the bottom. Loosen the C-clamps, then use the nut to release the tension on the spring. Once slack you will be able to spread the main leaf by hand, or with a little assistance from a ratchet strap or lever. With the main leaf in the shackles re-tighten the nut on the long bolt, replace the C-clamps securely on both sides of the bolt and remove the long bolt to replace the original center spring bolt and you're done.