will a 440 mag fit

Anything will fit if you've got a big enough hammer ;)

In addition to what Jason mentioned above, you'll also want to change out your torsion bars, beef up your rear springs for the torque, upgrade your brakes, and consider subframe connectors if you're planning to build up that 440. That much motor in a little A-body is an awesome thing, but it's got to be handled correctly.