The #0331 head refers to the casting number - I believe it changed sometime in 1999.
As I recall, the #0331 had bosses for mounting the coil rail for DIS, while the #0630 still used a distributor. The XJ did not get DIS until 2000 (I think...) so you probably still have the #0630 head. The number will be a raised casting, physically on the cylinder head, but I don't recall where exactly. I've got to work on that...
The #0331 cylinder head has occasional issues with deck cracks toward the rear cylinders, but a competent machinist can probably weld those up for you. I'm not sure if the coil rail can also be bolted to the #0630 head, so that merits further investigation (and is one of the reasons I'm collecting cylinder heads.)
While cracking CAN be an issue with the 0331 head, the odds in favour of cracks seem to be about 1 in twenty, I think, and many of those are probably smaller cracks that don't actually present until the engine is rebuilt.
If you suspect cracking, try a compression test and/or a leakdown test. If you do the compression test, make sure you A) duplicate the tests with the engine cold and at operating temperature (small cracks can close up as the head expands) and B) duplicate the tests while "dry" and "wet" (a wet compression test is done with about a tablespoonful of oil poured down the spark plug hole. Make sure you crank the engine over a couple times WITHOUT SPARK PLUGS when you're done, to blow the oil out.)
5-90