18 Spline 10" Clutch Question

I'm considering pulling the trigger and getting on the Passon A855 list. However, I'm running a 67 Barracuda with a 273, 4-Speed that originally came with the Auburn style 9.5" clutch. I rebuilt the engine and installed a new Borg & Beck style 10" clutch because all I had to change was the pressure plate and the clutch disk.

However, I'm concerned the A855 only comes with a 18-spline input shaft and my current transmission uses 10" clutch disk with the 23-spine shaft. I'm concerned that I will not be able to find a 10" clutch disk with the 18-spline input drive.

If I was using the more popular (and common) 10.5" clutch I'm sure I could easily find a clutch disk that had the 18-spline input drive. However, to use the 10.5" clutch requires me to change the bell housing, pressure plate, release fork and the flywheel.

After forking over almost five grand for the A855 I want to make sure I can find a 10" clutch disk with the 18-spline input drive and not have to spend another thousand dollars "upgrading" my clutch setup to work with the new A855.