The Gen 1 Small Block Chev gradually grew to 400cid. To achieve this massive volume the bores were siamesed and the conrods were shortened.The engine when produced in passenger car form had the same HP rating as the popular 350 but it made more torque making it better for the heavy cars it was used in .I would like answers to clarify or dispell all the rumours .
The general thought with the 400ci SBC is, when they are overbored by more than .030", you start running into overheating issues due to the Siamesed bores. This is not true. The overheating issue usually lies in the coolant passages (sometimes considered steam holes) which allow flow from the block to the head. These are very small holes which tend to get plugged up. Cleaning these holes out usually helps with overall coolant flow and help prevents overheating issues.
Another issue which may come into play with boring these blocks past .040" is wall thickness. Many of these blocks can accept an overbore of .060", but must be completely sonic checked before hand. There are reports of boring right through into water jackets with thin walls. A good wall thickness is said to be in the .150"+ range.
A third issue which needs to be addressed with these engines is to ensure they are honed with a torque plate installed and properly torqued (with all fasteners). When this is done incorrectly or not done at all, these blocks are known to have oil control issues. By placing the torque plates prior to honing (after being bored over), you are ensuring the cylinders are completely round. This really applies to any block which you are having bored over. If a machine shop tells you not to worry about it or it doesn't really matter, take your business elsewhere, because this shop is lazy or doesn't know what they are talking about.
Overall, the 400 SBC is a really good block. They do come with a bit shorter connecting rods (5.565" compared to 5.700" for the 350) than do most of the other SBC engines out there. I'm not too sure why GM built them this way, but it's okay. You can use the 5.7" rods (way more plentiful and cheap), but you have to ensure the piston pin boss is offset or you'll run the piston through the head (think irresistible force meets immovable object). Other than that, live long and prosper.
I've built quite a few SBC 350s and 400s over the years and have run them in different cars and trucks. I haven't had any different levels of reliability between then two. I've heard about overheating issues some people had, but I've never experienced any. The 400 ci engines definitely make more horsepower and torque compared to the smaller cube small blocks. It's not unreasonable to get 400+ HP from a SBC 400 with high flowing modern heads, intake and carb with the proper cam shaft. I'm currently building a SBC 400 with 6 inch rods and an internally balanced bottom end. This rod length provides a much better "stroke to rod length ratio" which according to David Vizzard reduces the amount of stress on the cylinder walls. I've only seen one issue on a 400 where piston rings were broken, and scratched up the cylinders, but this was probably and assembly error where someone didn't leave a big enough ring gap. There is also a lot of info out there that the 2-bolt main 400 blocks are stronger than the 4-bolt main blocks because of additional webbing in the casting design. You can read Grumpy's Writeup here. In summary, I think most issues people talk about with 400s are because they are pushing way more output than a 350 and expect the same results.