The nickel content is under the timing chain cover, some have 10 20 indicating the use of tin and nickel, if only one 10 or 20 is on the block indicates the percentage of nickel, 10 - 1% 20 - 2%
Tin helped the cast iron flow easier into the casting mold, nickel hardens the casting. 10 20 blocks go back all the way to the 60`s. Gm seemed to do this less in the later years, in the late 70`s the blocks were thinner and the use of tin and nickel was a lot less common. Most 80`s blocks I`ve seen are the same way. I`ve read the 307 small block never came with tin or nickel, but I have a 10 20 307 block out of a 1969 truck.