Torquelink,
I had a look into the same question a few years ago. The A343 is inferior to the A333 because of the add. operational empty weight (two additional pylons, centre landing gear installations being the obvious differences). Volume would be exactly the same, payload almost the same (about 64t, taking into account the loss of several percent OEW during conversion). Given those figures, I judge it highly unlikely that anyone would need a long-haul freighter (A340) with such "limited" payload capability; they´ll rather go for the A333.
Hope this makes sense to you,
J.V.