What is that little thing people say....NO REPLACEMENT FOR DISPLACEMENT!
But any way, look at it this way, if you want to be N/A, then run as much displacement as you want, why stop at 408 when you can go to a 418,420,427. Remember when you're running this big you can run these big heads (TFS'R, AFR 225, AFR 205) with a matched intake and have all the power you need and have no need to shift over 6,000 rpms, on pump gas kicken ***! Too have the same torque in the same power band with 331,347 you will most likely need a blower to keep it on pump gas.
Anyway, I say do what makes you happy, and have fun!
My vote is to make it at least a 408.
Good Luck