It sounds like you have both ends of the spring already attached, and your trying to compress the spring up to the axle. Yes?
If so, you might try disconnecting the shackle end, then install the u-bolts snug but not tight. Then connect the shackle end, using a floor jack to compress the spring into position. Lastly, torque everything to spec.
At least that's how I do it, and it seems to work out.
With my frame blocked up and tires off, I take a jack under the axle and let it drop to the springs level. I get the space between the pad and spring closed up, spin the nuts on the ubolts but not torqued, and then tighten things up when it's back on 4 feet.