There is allot of misinformation about early small block castings due to the various letters cast into them, it seems that people have a tendancy to believe that one letter or another offers something special....They didn't.

As mentioned, all 915 heads were the same castings regardless of the letter cast into them (BTW there were some with no letters at all). The "X" heads were a different casting number (ended in 894) but even those are nearly identical to 915 head castings, there's been allot of tests stacking one against the other, niether has ever been proven to be significantly better than the other (individual casting and valve job variables could easly erase any advantage by either).

If you want to buy a set of good early iron small block heads pick any of the above and go to a good machine shop, the end results will be the same no matter which cores they start with. If you want more bolt on HP simply buy a set of late model 308 castings, much better flowing head than any early head in stock form.