I had NAPA look up availability for the 9-bolt clutch cover last week. No listing at all. Not even NLA.
About four years ago I had a local rebuilder rebuild the clutch cover assembly. He botched the job but didn't charge me. He did the disc OK. I took the assembly to Chevs of the 40s and they exchanged it. I got a phone call a few days later from them asking if I really intended to turn in a completely rebuilt clutch as a core. I explained. I don't remember what they charged me.
I recently had the engine out and bell housing off and wanted to get a 6-bolt clutch but, BUT, Chevrolet changed bell housings, flywheels and clutch between '53 and '54 and the flywheel again in 55 2nd for the 12V starter. So, I planned to compare measurements between the height and diameter of the 9-bolt and 6-bolt clutches to ensure the 6-bolt clutch would fit. There isn't much extra room inside the bell housing when you look through the inspection hole, maybe 3/8". If the clutches were similar diameter and height, my plan was to have the flywheel drilled for 6-bolts when I had it resurfaced. NAPA didn't have an 11" diaphram clutch for me to compare and I didn't need a clutch at this time so I kept the existing clutch and will deal with the new clutch when the time comes.
If you can find a 6-bolt, 11 inch, diaphram clutch to measure against your 9-bolt and they are the same or close enough, you can have the flywheel drilled.
If you need only the disc, you can use an 11" in place of the 10-3/4". I have done this. The disc I got had more and beefier springs for a V8 and it always juddered slightly on take-up. I only pulled it because it got oil soaked but the judder was always a minor annoyance.
