More rumors but 600 HP N/A sounds pretty good.
http://wot.motortrend.com/1409_ford_shelby_mustang_gt350_to_get_5_2_liter_flat_plane_crank_v_8.html
http://wot.motortrend.com/1409_ford_shelby_mustang_gt350_to_get_5_2_liter_flat_plane_crank_v_8.html