I have never questioned that the X/J heads are good heads. at one time they were the best... I've owned a set on a hot lil 340 and ran them for many many miles and enjoyed the heck out of that car. they are quite sufficient for street use.
I only contest putting big money into retrofitting valves/porting/etc on X's/J's/(or other iron heads) when stuff starts hitting $1500 you are entering no-mans land.
only exception is the large intake/exhaust ports W2's maybe to extend your investment. or if you are building a factory correct resto.
if you just want to run them as-is for the street they are one heluva set of nice heads.
