Clutch size variations...

[martindc1]

Whats the deal with the different clutch sizes? We are getting ready to rebuild the engine in the 67 K20 and I have been looking at clutches. The parts store sites list a 10, 11 and 12 inch clutch. Mine is a 250 so all that's listed is either the 10 or 11. The right thing to do would be to pull the engine and measure the clutch, but can anybody give any insight as to what size clutch may be in it? The transmission is the sm420. I've noticed that the 11" clutch is a hair cheaper as well.