I am looking a a GM block. It has the casting number of 3970010. Were can I find the "Casting ID" number? How can I tell if this is a Z28 302?

You already found the casting ID. The pad next to the #2 bore must have a VXXXXDZ assembly date to be a 302. Chances are slim it is a 302 since that block was used for a ton of 350s also.

they used the very same block for 350's thru the 70's- some 4 bolts, most 2 bolts- so that's probably what it really is. my Monte has that very same block in it stock- but it is only a 2 bolter, and came with crappy smog heads and a 2 barrel carb.
personally, i'd only pay extra for a bonafide DZ bare block if i just had to have a "real" 302 block for a 69 Z and had more money than i knew what to do with.

Tyr to post back pic's of the VIN stamp w/ the letters at the end of it b/c there are some re-stamps out there b/c of the money involved!

pdq67