273 crank in a 340

Even a poly crank would work. But as said, it would need a serious balance job for heavy 340 slugs. Now if your going SCAT rods and light pistons, you may be closer than you think. The 273 ran a huge wrist pin, the larger bore LA 318 didnt to be roughly the same reciprocating weight. 340 rods and pistons were heavier so they took weight out of their bearing journals instead of adding weight to the counterweights.