The block casting number can be seen (sort of) from the top, behind #8 cylinder right by the bellhousing on the block, not the cylinder head. If the engine is as dirty as my old 400 in the photo below, you will need to use a scraper and wire brush to find the numbers. It can't be hidden by paint, it's raised above the block casting.

If it's an early engine, the block casting number will be up by the date. First pic is a '67 400. Second is a '71 400. On the '71 note its location near the oil filter adapter.

1970 400