[mikey]

You will need to be lucky to get a safe 0.060 out of a 400 block, its as much about porosity as thicness. The Merlin and Mowtown blocks as Wilf said are more capable and this is as much down to thir higher Nickel content which adds to the block material density and they are cast with a thicker wall also.

If it was me I would be looking at a replacement block and I think the 383 is a better proposition as whist the localised heating issues of the 400 can be overcome with a bypass it is only a partial solution. If you have the spare cash, do not dismiss the Dart or Motown Blocks where you can achieve 427 cu in safely in a small block configuration. Also the aluminium blocks are nice, but you are talking big bucks.

Good luck with it
Mike