Just found out that my numbers matching 390 block for my 68 cougar, has a 3-4 inch crack in the back water jacket area of the block . The machine shop said because of this damage the block is not rebuildable. Block has a casting date of 1/4/68. So what are 390 and 428 rebuildable core blocks going for these days? Also dose any one have one for sale? I am located in Denver Colorado and standard bore with a 68 date code would be my preference but I will take what I can get.