Let me take a stab at this until Jarret can correct me.
Top perch might put more of a bind without a modified Upper Control Arm to expand the angle of motion on ball joints. RPG UCA has a greater range of angulation that will reduce binding.
Rear springs, it is my understanding regular National rear springs have pretty much the same stock ride height Since I have not had a chance to install mine yet I cannot say for sure.
National HD springs will give a bit more "empty" ride height as where the springs are designed to carry a little heavier load. Not sure on specs but RPG can answer those technical questions.
The biggest advantage of National springs is the removal of stock lift block, reduces axle wrap up. Next advantage is a progressive rate spring that gives a better range of movement or articulation over the stock single main leaf another leaf and then somewhat of an overload leaf. Ford did an excellent job within the budget they had to work with, but there is always room for improvement.
National springs have 11 leafs total 2 main leafs which wrap around the front and rear piviot bolts. National springs have been around for Many years, I have had Natoinal springs on my old F-250 for about 10 years or more.
Ford off road racing used National spring as their spring supplier for many years during the "Rough riders" program and beyond
For a performance spring National springs are the way to go. If all you want is a taller ride height, well you would be creating a bigger problem with axle wrap up if you went to taller lift blocks. Same on the front with lift spacer blocks.
Maybe mid perch, and some 37" tires and a little trimming of the inner fenderwell
Hope this helps, any corrections welcomed as I do make mistakes Thanks Big Scott
