From the front edge of the front tire to the contact patch of the rear tire is a little over 67". So with tailgate down, if you have anything more than about 68" from the front wall to the edge of the tailgate, you should be able to fit the bike in there without dropping the midgate. (You could always load it diagonally if it's borderline).
I take it you never had your 996 loaded in the Avalanche?
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On the Chevy website, it says the box length inside (midgate up) is 63". So obviously, the rear tire will sit on the tailgate, but it should easily fit without opening up the midgate...
Have fun getting it up there, though! That tailgate is, what, 3 feet off the ground?