I agree that the stock shock locations will likely becoming the limiting factor in your articulation. Ford F-250 shock towers are relatively cheap and you can order up some long shocks (I run BBCS shocks from Poly Performance) and then place the towers in the correct location for the shocks.

If you are going to use the stock mounting, install the springs and flex the suspension without any shocks to measure the compressed and extended distances between the mounting points to figure out what size shock you need.

Harry Wagner
Harry Situations
4x4Wire's Toyota Section