The X heads are 894 number.
They do flow a bit better stock and
chambers are bit better aligned to block.
However, on a stock low compression 340
you will never feel the difference in performance.
I have done extensive research and testing on these
different heads for 50 years. If you gained 10 HP that
would be a lot. The headers would probably add much
more for less trouble and expensive. The intake and carb
if properly tuned and adjusted work quite well and aftermarket
parts would only add a small amount as well = although save some
weight.
OK, time for my enemies on this site to flame away and throw
rocks at me!!