Clutch disc 4-Speed Barracuda 1966
My first Barracuda was 67 with a 273 Commando and a 4 speed. weakest part of the whole car was that #$%*& 9-1/4" clutch! Hot setup back then was a Weber clutch but it was too spendy and I never did it.
My first instinct is to go to Rockauto. They do not list a 9-1/4" clutch for a 66 273, only larger. If you change the engine size to 225 /6 (3.7L), there is a 9-1/4" clutch kit listed for about $100. Note the 1"x 23 spline which matches what you have.
AMS AUTOMOTIVE Part # 05003 9.250" X 1.000" X 23T
With 9-1/4" Clutch; Flywheel Spec: FLAT (Only 4 Remaining)
You might consider trying to swap to a more common 10.5" clutch. You'd need a different bellhousing and flywheel for sure as the 10.5 does not fit the 9-1/4" BH.