Did you use 400 head gaskets? If you did, did the heads have the corresponding steam holes drilled in the heads? Classic 400 Chevy problem.
The 400 SBC has 2 small holes in the deck surface between the cylinders to vent the water jacket between the siamesed cylinders (on the 400 there is no water jacket between the cylinders like on a 350). There are matching holes in the head for the block to vent. Only 400 heads have this from the factory. If you are using aftermarket or 350 heads, the small holes need to be drilled. Use the gasket as a template. Before you put the heads back on, check the heads and block for warpage. If you didn’t have this before, you might now. Good Luck