Answered your other thread about the EGR and what heads are what. The P heads will likely have problems with the stock headers. Also, the stock headers are trash and should be swapped out ASAP. No better time to do it than now.
For springs... the E7 springs are barely enough for the stock cam, if you have an aftermarket cam in there they should be swapped out regardless. The Explorer springs are even weaker. You also probably don't have the right length pushrods if you just threw a cam in there and went. You have to properly measure pushrod length any time you make a change in the valvetrain.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.