Depends on the goal,a "stock mustang" can mean most anything. I'd build a strong streetable 408ish cube small block,but for a few $$$ more you could get the same result or better with a big block. Your choice. How bout a stock iron headed pump gas 446" bb in a 3700lbs run mid 10's and drives everywhere on pump gas? http://www.jsiskphotos.com/Cars/Holly-Sp...8&k=bqrX4CH