XL guns in a spinal mount... Terrible idea.
For one, forget tracking... You have none. Zero. Nada. The ships turn rate is your tracking. Anything able to get any sort of velocity up in transverse can avoid your gun by virtue of your ship reacting to the movements, rather than pulling lead, the firing barrel always being in line with where it is pointing, which is "trying to catch up to the target". Worse than that, the singe gun by itself is completely outgunned by a standard battleships full rack of weapons, since XL guns aren't, on their own OMGWTFPWN powerful. That's a dreadnoughts siege mode at work. The XL gun cruiser would be dead before it got enough shots off to make a difference with it's single, underpowered, weapon, lacking the survivability of a battleship or even a battlecruiser.
Also, cruisers are already vulnerable to small, fast targets. Vulnerable, not helpless. That armament scheme would make it completely helpless against anything that comes after it below a certain size... Fast cruisers, frigates of any sort, and drones. Too narrow, too focused, too vulnerable. There are already counters that have other uses too, as was pointed out, which would be, and are, far more effective. What this all amounts to is that the small, vulnerable "cap-killers" could be wiped from the field by frigates, drones and fighters in short order, while posing little real threat to a capital ship to start with.
Finally, and most decisively, there is no code to do it in EVE. Doomsdays were supposed to be spinal mounted at one point, and CCP couldn't figure out how to make it work.