This 66 will be mostly for the drag strip,so aluminum heads are lighter and can also be made for higher hp ratings.I will be selling the X heads to recoup some of the cost for alumi. heads.
Ok, it always been my understanding that outside of weight, iron will out perform aluminum if all other factors are the same. Well the alumium you can run a lower octane since the motor thinks is has a point less of compression. I do guess that the X may not be able to get the same specs out of as with the aluminums.