Clutch Sizing Questions

[Gibbelstein]

So it's time to replace the clutch on the '69. I have seen an array of sizes, etc. So, I have a few questions:

First off: Which do I need for a '69 c10, 1/2 ton, 2 wheel drive, 250 I6, 3 on the tree, power nothing.

Secondly: For future user's search purposes, if anyone knows a formula or resource that searchers can use in the future, would you mind sharing it? Maybe this can take the place of the multiple "what size do I need" threads I found when I searched.

Third: I went by the parts store and they said they listed 10", and 11" and they couldn't get one of them at all. Do they use the same pressure plate? If I had one and they only sold the other, could I use the one they could get? I.E. if I have 10, could I use the 11 without a problem (or vice versa)?

Thanks in advance for any help I can get.

Chris