[Jonesy]

The 'high tin/ high nickel' story is actually just a myth, John Hinckley posted about this over at CRG:
"And, as it turned out after further research with the Saginaw Foundry (now called Saginaw Metal Casting Operations, part of the GM Powertrain Division), the old story many of us were led to believe about the 010/020 describing the tin/nickel alloy turns out NOT to have been true at all, although the magazines thought it was true and continued to publish the tale, and still do today.
Actually, the "010/020" cast into the front bulkhead under the timing cover turned out to be simply the identifier for the foundry pattern for the front bulkhead, which was shared by the 3970010 (350) and 3970020 (307) blocks; it had nothing to do with the iron alloy, which was never altered for any particular production block (although the alloy was altered for some later low-volume GM Performance Parts over-the-counter "Bowtie" blocks)."

Some people still do not accept this.