IIRC, the standard duty was 9 1/4. HD for the Commando 273 (starting in 65 of course) was 9 1/2.

You cannot to my knowledge put anything bigger in the stock 1964-6 273 bellhousing.

Alternative is to do what I did with my 65 Barracuda: swap in a 10.5 bellhousing and flywheel from a 340 A body. You'll have to modify the clutch linkage a bit, but the bellhousing and flywheel fit a 273 and its original 4-speed just fine.