Listen to jmk97GT! I saw a guy @ torquefreaks in Portland put down 680 rwhp with a kenne bell 2.9L. You could build a DOHC 4.6 with a Kenne Bell for half the $ and time of installing a bow tie.
I have a SOHC 4.6 and I really don't know that much about the 90's DOHC's. Do you know if they are aluminum blocks? Do they have steel cranks and forged pistons?