To get the correct spring length, measure from the center of one pin, to the center of the other pin, not overall length. Measure the width of the spring. And then count the number of leaves. Replace the U-bolts and nuts too. If necessary, just cut the old U-bolts to get them off. To get the right U-bolts, you may need the axle
tube diameter, and the U-bolt's overall length.
Old leaf springs can sag over time, and they can also snap off if they are deeply rusted. Good idea to replace them.