While the X head is the most sought after head for factory iron, in general, all small block heads flow about the same and will port out very similar to each other. I would not sweat what you have, there a good head...
a stock X-head will flow about 220cfm @ .500" to .550"
a typical stock 360 head will flow about 180cfm @ .450" lift.
273/318 stock heads are way worse yet.
(FLOWED @ 28"Hg)
