I'd say an evolved 4.6L would be nice. Reduce the stroke and increase the bore, add a true 2 staged intake setup, create a full roller valvetrain setup, base the system on the 5 valve heads, and joust up compression to about 10:1-10.5:1 or so. This design will not only reduce the tab for repairing the motor, but it'll significantly increase performance to money potential.