I'll admit that I'm both massively confused and massively frustrated. Today, I had the frame supported by jack stands in front and behind the leaf spring mount points. I had a bottle jack under the diff and the wheels/axle were just slightly off the ground.
Here are two photos of the setup. I already have the welded on bracket (with two holes for the spring to slip into)
I took the mono springs off and installed the multi-leaf springs, first at the shackle and then the front mount. Since I don't yet have the longer u-bolts necessary (they arrive tomorrow) I just wanted to see how close to fitting I could get the springs. Using a bottle jack I compressed the springs upward towards the axle. If I had the longer u-bolts I could have partially tightened the u-bolt nuts to get the lower shock bracket to get closer to the axle, but the nub that needs to mount into one of the axle holes were still not aligned, but far closer.
The next time I attempt this (after the longer u-bolts are here) I will likely remove the wheels and support the diff with my rolling hydraulic floor jack so that I can attempt moving/adjusting the position of the axle over the springs. Which of the two holes should I be aiming for?