318 crank into 340? pitfalls

Don't forget heat treat. "Heat Treating
In addition to materials, and casting or forging techniques, heat treating can greatly impact the strength of a crankshaft. Nitriding is the most prevalent method of heat treating used in aftermarket cranks, where ionized nitrogen is vacuum deposited onto the crank surface in an oven. By penetrating .010 to .012 inch into the metal surface and changing the micro-structure of the steel, surface hardness is doubled from 30 to 60 on the Rockwell scale, and fatigue life is increased by 25 percent. The OEs usually favor induction hardening over nitriding, which results in deeper penetration into the metal surface (.050 to .060 inch). This process uses a magnetic field to heat the surface. “There are pros and cons of both methods, but nitriding is most common in the aftermarket,” explains Humphries. “Induction hardening is more localized, whereas nitriding treats the entire crank at once. However, induction hardening penetrates more deeply, which enables turning down the journals once or twice during rebuilds before having to heat treat the crank again.”