Personally I feel that five leaf springs are too stiff for the weight of a Mustang. It makes the back buckboard and unstable on rough roads IMHO. I think the five leaf is the four leaf with the extra bottom leaf added, but I could be wrong, so I would take that leaf off and see how the car sits/rides. The only cars I have seen to really benefit from the 5 leaf springs are drag only cars with cal-tracs. I have the original 4 leaf springs on my Shelby with the Global West bushings and it is as stiff as I would want on this light of a car. You can always put the leaf back in if you don't like the results.
To get the leaf out easily use two very large C-clamps (or the like) and put one on each side of the center bolt. After removing the center bolt slowly release the clamps until you get the tension off of the leaves. After removing the bottom leaf then you use the C-clamps to pull the other four leaves back together. I use a long piece of threaded rod in place of the center bolt for reassembly purposes and remove it and replace the proper center bolt after I get them all clamped in place.
It is up to you, I have used lowering blocks but they are not the best solution because they amplify axle wrap but on a street car they are useable. You can try either the blocks or removing the leaf...or both.