318 crank into 340? pitfalls

What year is your 340? The '68-'71 and early '72 340's had forged cranks, so you could be going from a stronger forged crank to a less strong cast crank from the 318. And it this is a later 340, then the comments on the damper and flywheel change.

I personally would not be concerned with turning the crank down .040".... these cranks, forged and cast, have a reputation of being quite strong and durable. And your build as described is not at a 'gnarly' level of torque and HP, so that makes it even safer to do. The crank journals' cross-section area will stay at 96-97% of original with .040" undersize. And folks turn these cranks down .030" regularly and even .125" if using a 2.000" journal Chevy rod.

Adding weight to the crank tends to be a more-$$ thing to do to a crank when balancing; I've read reports of $75 to $200 to add weight. With the L2316F's, the 318 crank is not going to balance without adding weight. The L2316F's are stock weight, so if you used them and the 340 crank, then any re-balance would be simpler.