crankshaft help

I'd just buy a used forged 340 crank. The cast crank 340s were externally balanced. That started in '72 but that year was mixed cast and forged. I always internally balance and the forged crank will do that cheaply. The cast crank you may be spending more if you want to internally balance it... depending on your piston choice. Any 273/318/340 forged crank will work fine, but the counterweights on the 273/318 are smaller than the 340 but if they are forged I think there's enough meat to get them balanced without adding Mallory. 350/400 hp is a stock lower end, performance valve job, and a camshaft in a 340. No need to get exotic.