411: if you gotta use a jack or something to push the spring DOWN, you got a problem. Were you doing one side at time, or trying to put one longer shackle on one side while the other side was still stock and attached?
If I were doing what you are, putting on new shackles, I'd do this:
1-jack up truck, jack stands to the frame, let front axle droop, remove tires if they hit the ground before it's hanging only by the springs.
2-disconnect shocks at axle housing, and extend as far as possible, see how much farther below their mount on the axle they extend to, this is as far as you can lift the truck with out new shocks. Leave them dissconected.
3-use floor jack to hold axle houseing, and disconnect both shackles, and allow that end of both springs to drop the ammount needed to install your new, longer shackles. Sway bar may also cause some binding, or restrict movement in some way, you'll have to check that also.
4-if all is well, and your new shackles fit, bolt 'em up, let the axle housing hang again, and re-check the shocks. If ok, re-install, make sure all bolts are good and tight every where, get it back on the tires and off the jack stands, and see if you like your new ride hight....
Hope this helps...
Greg