I've posted what changed and when with the 6-242 engine on several boards, and I think this was one of them - you may want to check.
You'll find your options will open up, however, if you look for a short block and clean up the head. Every 6-242 block will accept every 6-242 head, and even the 1975-up 6-258 uses the same screws and screw pattern as the 6-242 (earlier sixes use 7/16"-14 screws, which requires some small adaptation to make them work.)
If you use a RENIX block, just ignore the drilled/tapped hole for the knock sensor (or just put a screw in there and call it good - the hole doesn't communicate with any fluids.)
There were changes to the block (internally - casting revisions) ca. 1991, 1996, and 1999 - but that won't have anything to do with accepting a cylinder head. (Revisions were usually due to NVH - Noise, Vibration, Harshness - concerns.)
I have heard there were differences in mounting between the TJ/YJ and the XJ/MJ/ZJ/WJ, but I can't swear to anything. However, they're probably adaptable as well, I'm just not sure how much effort would be involved.
The 2000 XJ 6-242 should have the #0331 head - which was used 1999-up in the TJ and WJ, and 2000-2001 in the XJ. However, there earlier #0331 heads (through about mid-2001) hare the ones that crack, they've ogt about a 50/50 chance of doing it. The heads were revised ca. mid-2001, the revised head carries a cast-in "TUPY" mark on the upper surface.
Typically, the cracking occurs ~150-180Kmiles, and if they crack, it will be on the upper surface, roughly under the oil fill cap (between the #3 and #4 cylinders.) Any of the later #0331 heads will replace the earlier ones, so there are more heads that are less likely to crack than heads that are likely to do so (make sense? I wasn't sure how to phrase that.) Manifolds and attachments will readily swap.
"recon" (sic - reckon)(tm) "hihgly"(tm) "seceed"(tm)
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