If you are talking about re-using the u-bolts you have in there now, do not re-use your u-bolts please. u bolts are a torque to yield application, meaning they have but one life. i know it looks like you could back off your nuts about an inch and insert a block, then re-tighten, but this is dangerous. especially if 800# of tools could fly through you.
if you are talking about getting new u-bolts that are the same size, no i don't think you'd have enough thread under the nuts.
alternatively, can i ask about your leafs? if you have that much weight in the back, i would actually recommend going to a j-yard, and buying a pair of leafs from a four door explorer 91-95. take that leaf pack apart, and add leafs from your bronco ii leaf, to make a mega leaf. people have done this, and say it lifts as much as you want (add more/less leafs, and explorer springs alone lifted mine 1 1/2 " over stock). they also say it rides as rough as hell, but if you have that much weight in the back, it might be considered heavy-duty.
cheapest and easiest - rear shackle replacement. you can get about 3" of lift doing this. you can lift it 1" as well.
when you do the shackles or springs, please use new bolts every time. which would you want, bolts that look like this,
or bolts that look like this:
