well the twin turbo mustang is going to use the V6 engine and not the V8. but there are about three different rumors
1.) Drop the V8 and have a twin turbo V6 as the new "GT"
2.) lose the 4.6L V8 and pick up a whole new 5.0L
3.) lose the 4.6L V8 and use the 6.2L engine found in their trucks
i doubt #3 and #1 will happen because ford has never used a big engine like chevy and dodge to compete with them and i doubt they will now and they created a new V6 4 valves per cylinder making 270hp and 270tq and that will be the new V6 and the GT will probably get the 5.0L engine. so your best bet is just to wait it might not come out till 2012 but still wait and you will be happy trust me