340 wall thickness & deck height

The first question is more one of opinion. He may feel it can go that big. Others may not. In terms of my minimums - for a std stroke application, a minimum of .150" on the major and minor thrusts would be about it. For a stroker up to 550hp I'd be looking for a minimum of .180, with no minimum less than .090 for the pin axis. One or two cylinders that are badly shifted or thin can be fixed with sleeves or moving the bore centers to address the issue(s). If it's more than that, test another block.

Decks are always off, and always tall on factory original blocks. I will always square deck anything that matters. That's not "decking". That's square decking, meaning using the proper fixture to ensure the cylinder bores are exactly perpendicular to the crank axis.