I just went out and measured my C. The exhaust hole is the same diameter as the air cleaner hole. It seems like some of these tractors had a hole big enough for the muffler body as original equipment, not just the muffler pipe. Maybe that was just the D series. I don't know. If the manifold has been, or will be replaced, that might change what muffler will fit. All that to say- you might want to consider all that before making a final decision. Looking at the same model, but different tractors you will see why they made the holes seemingly so big- sometimes one edge will be close to the pipe. So custom fitting yours as JMS says is a good idea.
I put underhood mufflers on my B & C. to avoid low branches as I mowed around with a belly mower.