I have no idea about that, cause I just dont know...
But, I beleive 5 speed Mach 1's have a forged crank, autos cast. So possibly the other DOHC 4.6's were regular cast cranks...
Simply put though, no. If anything is forged its just certain 4v 4.6's crankshaft. The ones running forced induction on here and the ones that really know there stuff say 400rwhp is the edge of the safe limit on a stock internal 4.6 2v, I beleive its around the same for 4v's (non 03/04 Cobra).
I asked my friend earlier. He is a 96-98 Cobra fanatic. All those Cobras had a forged crank, and it is capable of handling big HP. The rods and pistons were PM and cast. I don't know about '99 up 4V. I know they redesigned a lot of things for 1999.