I'm with Harbormaster on this. But perhaps the issue is price. The 18.5 WSM is great but not cheap at $300 and they don't come up too often on CL. However were I to do the conversion I would be inclined to start with a CL 18.5 WSM and shorten the legs or fab some short legs.
Is there a particular reason to use the JJ instead of a kettle? Kettles are much more available on CL. Just looked at the Weber site and I see that the JJ has a single daisy wheel bottom vent vs the ash sweeper so it would provide better temperature control (unless you find an older 18.5 kettle with daisy wheel bottom vents. The WSM has always had the daisy wheel vents AFAIK.)