It depends where you are measuring from. I'm pretty sure GM calls for the outer face diameter to be about 176mm max and the larger area where the balance holes are drilled to be about 182mm max.
176 = approx 6.9
182 = approx 7.2
So pretty much expect the very widest part of the crank pulley to be around 7.25" with rust, etc.
EDIT, I see you want it for belts. I read that at first but got a bit carried away and forgot as i went to type this message. Just use 7" as the others said above.
[This message has been edited by Jncomutt (edited 05-06-2008).]