* Bolt Patterns: While both are small-block Chevy, the bolt patterns on the heads are different. 350 heads have a different spacing between the head bolts than 400 heads.
* Combustion Chamber Size: The combustion chamber size and shape are different. This will significantly affect the compression ratio, potentially leading to engine damage if not accounted for.
* Valve Size and Location: While some overlap might exist, the valve sizes and locations might not align perfectly, affecting performance and potentially causing interference.
* Intake Manifold: You'd need a manifold specifically designed for the 350 heads, which might not fit perfectly with other components on your 400 engine.
While some machinists *might* be able to modify a 350 head to fit a 400 block, it's a complex and expensive undertaking. It's almost always far easier, cheaper, and more reliable to source the correct heads for your 400 small-block.
