That flywheel can be used on both a car and truck with a 235 engine. A 9.5 clutch was used on a 6 cylinder car. Most 6 cylinder trucks
used a 10" unless it was a HD application. That appears to be drilled and tapped for a 10" clutch. An 11" would have the bolt holes
closer to the ring gear.
You want a flat diaphragm clutch with the long bearing. Most parts stores will try to sell you something else but
it won't work.
Fred

BTW. Not all Tri Five Chevy cars are Bel Air's. The car that was most produced and sold in those years was the 210 four door sedan.
There were also more 150 models sold than Bel Air's. A wide assortment of engines and transmissions was available for all models.

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine