[nxtruck]

You could always stroke the 350 you currently have, making either a 383 or a 396, depending on the crank you use. A 3.75 stroke crank will make a 383 with a .030 overbore and a 3.875 stroke will make 396 inches with the same .030 overbore 350 block. Generally, the 350 blocks are stronger than the 400 blocks and will tolerate the extra power and stress a little better.
If you're not looking for huge power #s with the 400, as long as it checks good at the machine shop ( sonic-test, core shift, etc. ), I would stick with a 406 to minimize any reliability problems. I run a stock-block 434 right now and, while it has been trouble-free so far, I wouldn't build another 434 using a factory block.
Just my opinions...