The engine in question will be anywhere in displacement from 308-503. I have no clue what the original displacement would be. I've even heard tell of a 707, but I'm not sure if that even uses the same block casting? Do the big GMC 6's have the displacement on a pad by the distributor like the smaller 6's?
As mentioned above, I saw that on ebay as well. Nice find! Take more pictures!
Jeff
