First off make sure you know how to I.D. a 400 block. Just because it has an externally balanced damper and flexplate is not enough, get a casting number. And even better a Suffix code.
I have had the same 400 for almost 15 years now. I had it bored and decked. Looking at the ridge in the cylinders I couldn't believe it cleaned up at .030. It has also had a crack in the deck from a steam hole to a bolt hole, been there for 7 years and has never been a problem, just stay away from cracks that go into the clyinders.
I have also seen people put 350 heads on 400's not knowing to drill the steam holes and overheat the block so bad it cracks. My 400 has never run hot, even at .030 over.
Heads should not be much of a concern because any 350 head will bolt on after the steam holes are drilled. You usually won't run into any unusual problems with any other parts, the crank stroke and rod length are different than any other small block but they are made of the same materials.