While I am not trying to initiate a big block versus small block argument, I personally subscribe to the "build enough HP and/or torque approach" whereby the weight thing becomes a "non-player."
It has been reported that you can take approximately 140 pounds out of a 460 (via a 521/545/557 build with
aluminum heads, intake and water pump). This will provide a motor weighing less than @600 pounds making in excess of 750HP (at the flywheel) and a nearly FLAT torque curve between 2500 RPM and 6500 RPM.
Quite frankly Olie, I don't think you need to take out any additional weight considering your stated use (bracket racing/cruising) of your Mustang, even with that iron-headed 521
.
Just my $.02
BT