Here is a ZL-1 block that is similar to the one I have, and has similar attributes to what is described in this thread.
I will be watching it to see where it ends up. With that casting number, it can be considered for use as is in FAST/purestock drags, as an FYI.
http://www.ebay.com/itm/1969-ZL-1-ALUMIN...=item256bded370