the stock block can handle gobs of power, but any 4.6 besides the 03-04 cobra, the factory internals will not live past the 450 mark. anything past 400, u better have a good tune and try not beating on the car hard repeatedly or u will have brand new windows.
I have read that the Teksid aluminum bolcks, with basic machining, can handle upwards of 1,000 HP. Just remember those are the blocks Ford had cast in Italy. They are not hard to find in the old Lincoln Mark VII cars.
Jim Wyche:Senior Citizen. 1992 Mustang GT with a few mods. See webpage http://web.mac.com/jwyche or my garage. 2002 Mustang GT with a few mods. Collegedale, Tennessee.
my buddy matt has made about 380 in heat at the rear wheels. that should equate to nearly 450 flywheel hp. thats on a real conservative 110k+ longblock. i wouldnt push it too much further than that though