How big a clutch for how much power?

One option you left out is the 10.95". It will fit where the 10.5" will, because it has a scollaped pressure plate. I would get the casting number off your bellhousing and look it up and see what it says fits. Put the biggest clutch that'll fit.
That's what I had in my 68 383 Coronet... she held Good.....aka the 11" Scalloped