Well, it was bound to happen. A modeler on the Hobby Heaven Message board is in the process of making a large scale allison, both in 1/6th and 1/8th.
He is currently posting on the board to see what scale would be most popular and so far 1/8th seems to be the scale of choice for this beast!
Link to his Fotki site:
http://public.fotki.com/gkulchock/allison-v12--18-and/
And the Hobby Heaven Posting:
http://wwwboard.modelcarkits.com/index.pl?read=1025618
Will keep all posted on progress as he posts it on the board