The very early blocks, (mid year '71 casting) had thicker main webbing but........SONIC CHECK any block you want to use. Some of the later castings such as the 77-78 seem to have consistantly thicker cyinder walls. In all fairness, any 400 block should work fine as long as it sonic checks alright.
I've got a 7-31-71 400 casting with the thick main webbing I'm using as a doorstop. HemiRam from this site wanted to build a 400 stroker a few years ago. I told him I'd sell it if it sonic checked alright. 4 cylinders were real bad, one checked at .078 thick, this is a standard bore block too. So, as I said, sonic check any block before dumping any money in it. Dave