I had never heard of this before. Google is your uncle anyway:
4.6 Liter Ford Engine Cylinder Blocks
[SIZE=3]"Our recommendation is that the early block (the Italian Teksid) is the best foundation for power levels above 900 hp, although the later block certainly should be adequate for
moderate power levels of up to 600 hp."[/SIZE]
Damn, I like their attitude. "Moderate" power levels up to 600 hp. If I asked for an old school 289 making even 500 BHP back in the day, all I would have got would be eye-rolling and "good luck with that!"