I'd call your local engine builders. Some have stacks and stacks of blocks for sale. I purchased a 68 400 block with a 4 bolt main X coded for about $700. Whenever I need anything BOP, I call all the local transmission, machine and engine builder shops and ask what they have on the "junk" pile. I've gotten several TH400's, blocks and other parts this way. They are usually happy to get rid of them at a fair price.