You can use an aluminum block (and head) motor with a supercharger, although cast iron will take more horsepower.
I dont know what the highest hp Ford V6 is off the top of my head, probably something they put in concept car or possibly the newer Mustang engines?
I'm guessing the Tbird motor you're speaking of is the 3.8L? I'd stick with the newer Mustang engine. I dont know the differences between the two but i do know that the older 3.8L Ford engines had horrible problems with head gaskets without a supercharger.
More. BTW one of the Shelby Mustangs that were just released have 350. More than that is definately achievable but obviously the more hp you pump out of any engine the more likely you'll have problems.