You're not limited to Cobras if you're looking for aluminum blocks either, although they usually get the best cranks, if not rods and pistons.
Lincoln Mark VIII had them (4V, very similar to 96-96 Cobras) and you can find those at pick-n-pulls for almost nothing. Also, later years of the Explorer had aluminum block PI 2V, Lincoln Aviators had 4V aluminum motors but would need a shorter car intake, Mach1 and Maurader had aluminum 4V motors similar to 1999-2001 Cobras and of course, 2005-2010 Mustang GTs are all aluminum blocks.
There are several different castings with reputably different levels of power handling, but within the realm of what you and most people are doing (like, less than, I dunno, 1000HP), it doesn't matter. All the blocks are way stronger than what 99% of people will ever need and they should all be good for at least 7000 RPM N/A with stock internals.