Hello I'm new here looks like a nice site with some great info.
There are some inherit problems with the smallblock 400. I wouldn't say they are bomb proof. The rods on the 400 are really short 5.565 inches and cause some nasty rod angles that can cause for easy rod failures. Just about everyone I know that has twisted a stock rod 400 over 5000 rpm too many times has put a rod through the block or pan. They do make some great torque, which is great for the street. But, you can't twist em up.
If you want one to last and make some good power I would suggest putting a 6 inch rod in it. With a good quality 6 inch rod 7000 rpm isn't out the question. The 5.7 inch rods in which are in all of the other small blocks are pretty good also but just watch how many rpm's you twist it up. I would keep it under 6000 rpm's with a stock chevy rod if you want it to last.