I purchased my rotisserie parts, and pieces from
www.onegrill.com/ . My DB used a 37'' spit rod. The JB would probably need a 43'' if installed the same way. I can't remember if the centerline of the spit rod is 6'' below inside top of grill or the top of the spit rod is 6'' below inside top of grill. Either way it is a 1/4'' difference. I did not factor in clearance from the grill grates because I usually remove them when using the rotisserie. If I remember correctly the grate, and ceiling clearance is almost identical.
I installed the spit rod with a hole on the hopper side, and stack side of the grill. Other members have avoided drilling a spit rod hole on the hopper side.
One member used a magnet attached to the inside of the grill with a hole drilled in it to accept a bushing or just the rod itself I can't remember.
The other way I have seen is using the supplied spit rod BUSHING SUPPORT MOUNT, and appropriate supplied bushing (part of motor mount rod kit) mounted on the inside of the grill. Still need to drill 2 holes in hopper side to fasten bracket with this method.
As stated above I drilled a 5/8'' hole on each side, and used a STEPPED SPIT ROD BUSHING.
Hope this helps a little.