6 covers is a 5.9L. My grandfathers motorhome has a 8.3L 12 valve, and it is definately only 1 valve cover on it. You can get adapter housings to use most any SAE 3 or SAE 2 trans set-up I believe for it.
Umm don't know what 8.3l you guys are looking at but Cummins made individual valve covers for the 8.3. They just were a little larger than the ones on a 5.9
To the OP, get your tape measure out and tell us how long the block is. Just the block, from behind the gear housing to the front of the flywheel housing.