cbmax,
Be careful about this block. There were no 406 blocks with 64 casting numbers or dates. There were, however, lots of 390 and 352 blocks with 63 and 64 casting numbers that had the bosses for cross bolts. I've run across a bunch of these old 390 blocks that someone has installed 427 caps and cross bolts, then tried to pass it off as a 406 or 427. As has been said, the casting numbers will tell. The 406 casting numbers all look different from normal FE casting numbers. They are smaller letters and are right up at the front of the block on the drivers side. Should be C2XX or C3XX if it is a 406. It will also be a solid lifter only block and should have a
oil pressure relief valve exactly like a 427 center oiler. It will also have the letters HP cast in the lifter valley.
There were, though not many, 406 blocks that were cross bolted by Ford. It could be that this is one of them. Steve Christ's FE book is a great resource for casting number research.
The price is also a bit steep for the bare block. You can buy a useable 427 block for that much. The 406 block makes a VERY nice engine when built with 428 crank and pistons.