Sorry I thought the 240 could not run the late 5 speeds (oh yeah your not in a 240 duh! my bad )But seriously have you looked into that? The truck rear diffs are plentful and cheap. I heard that the OD box set likes the taller gearing and they are also a lot easier/cheaper to get. Sam N said somthing to me about that last year as an alternative to trying to fix/replace the comp boxes (and trick parts are available if you want to tweak/rebuild one).
I think this is what Don Ahrens did with his car (not IT but very close in the drive train)