* Crankshaft: The key change. A 350 uses a 3.48-inch stroke crankshaft. A 355 uses a 3.75-inch stroke crankshaft. You *must* replace the crankshaft with a 3.75-inch stroke unit.
* Connecting Rods: While sometimes you might get away with using the original rods, it's generally recommended to replace the connecting rods with ones designed for the 3.75-inch stroke. The longer stroke requires longer rods for proper piston-to-wall clearance and to prevent rod-to-piston contact at bottom dead center (BDC).
* Pistons: You'll likely need new pistons. The piston's compression height must be correct to work with the new crankshaft and connecting rods. The choice of piston will also affect your compression ratio.
* Balancing: This is absolutely crucial. After installing the new crankshaft, connecting rods, and pistons, the rotating assembly (crankshaft, connecting rods, pistons) must be dynamically balanced. An imbalance can cause excessive vibration and premature engine failure. This is best done professionally.
* Other Considerations:
* Clearances: Verify all clearances (piston-to-wall, piston-to-head, etc.) after assembly. Incorrect clearances can lead to catastrophic engine failure.
* Rings: New piston rings are necessary.
* Bearings: Main and rod bearings should be replaced.
* Gaskets and Seals: All gaskets and seals should be replaced.
In short: Building a 355 from a 350 is a significant engine rebuild, not a simple swap. It requires machining, precision measurements, and careful assembly. Unless you have significant engine building experience, it's best to have a professional perform this work. The cost of parts and labor could easily exceed the cost of simply buying a used 355 engine.
