Here's why it's complicated and what's involved:
* Main Journal Diameter: The 400 small-block crank has a larger main journal diameter than the 350. This means the 350 block's main bearing bores will be too small. You cannot simply press the 400 crank into a 350 block. The bores need to be bored out to the 400's specifications. This requires specialized machine work and precision.
* Oil Passage Differences: While not always the case, there might be subtle differences in the oil passages between the 400 and 350 blocks. This could lead to oiling problems if not properly addressed during the machining process. A professional machine shop will need to inspect this.
* Connecting Rod Journal Diameter: The 400 crank also generally has larger connecting rod journals. This means you'll also need 400 connecting rods to match the crank. Using 350 rods will result in catastrophic failure.
* Potential Block Weakness: Some 350 blocks might not be structurally sound enough to handle the extra stress of a larger-displacement crank and the increased power it allows. A machine shop will assess the block's integrity before undertaking the machining process.
In short, it's not a simple swap. It requires professional machine work, including:
1. Block Inspection: Thoroughly inspecting the 350 block for cracks and weaknesses.
2. Main Bearing Bore Machining: Enlarging the main bearing bores to accommodate the 400 crank's larger journals.
3. Possible Oil Passage Modification: Addressing any oil passage differences.
4. Line Boring: Ensuring the main bearing bores are perfectly aligned and parallel.
5. Appropriate Crankshaft Selection: Choosing a 400 crank in good condition.
6. Matching Connecting Rods: Using 400 connecting rods.
Instead of this complex modification, consider these alternatives:
* Using a 350 Crank: This is the much simpler and safer approach. You gain the benefits of potentially building a higher-performance 350 without the complexities and risks of changing cranks.
* Starting with a 400 Block: If you're aiming for a 400ci engine, begin with a 400 block. This avoids all the machining challenges described above.
Unless you have extensive experience with engine machining and rebuilding, this is a job best left to a professional machine shop. The cost of the machine work and parts might outweigh the benefits compared to simply building a high-performance 350 or starting with a 400 block.
