Based on the casting numbers, this was in a mid-70's GM truck, but I don't really know for certain. I have no idea of the diameter; I was hoping that someone might know of a simpler solution than pulling the engine. I saw a recent article (don't recall where) that showed how to count the exposed teeth through the starter hole to determine which of two possible tooth counts you had (on a hemi, I think).

I should have added that this is backed by a TH-350, not a clutch.