What was the size of the stock clutch with a 273, 4 speed. I think 9 1/2. with stock flywheel and bellhousing how big of a one will fit??? Or what mods to use a larger one.

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.

I put a 10.5 pressure plate/disc in the stock bell with no changes other than using allen head bolts to bolt the hat to the flywheel so it would clear. I would go to the /6 site www.slantsix.org & ask as it was decades ago when I did it but I am pretty sure I stuffed it into the stock bell.