67' 318 crank in a 69' 340

- The bobweight using the 340 pistons and rods is around 2326 grams.
- The earlier light rod 318 bobweight is around 2147 grams, and that is what the 318 crank is balanced for; that is a difference of -179 grams. So 340 rods+pistons with a 318 crank will be waaaay off-balance.
- Put the heavy 340 pistons with the lighter (early) 318 rods and the bobweight will be around 2271 grams. That is still 124 grams heavier than the stock 318 bobweight... No way will it be balanced, not even close.

As you can see from the above, the 318 cranks are counterweighted for a significantly lighter bobweight; that is why the idea of not need rebalancing is incorrect.

OP, if you want to rebalance the 318 crank for the heavier 340 rod and piston package, you will have to:
- drill out some of the 2 end journal pin material (like in a 340 crank)
- and/or add weight to the counterweights (And even if you do the above, you will still have to add weight.)
Either way it will be pricey to balance this combination of 340 pistons with the 318 crank.

I would just bite the bullet and drop in some lighter KB 340 hypereutectic pistons and balance the 318 cast crank to that. If you need help on numbers for the bobweights of those pistons with the 318 or 340 rods, let me know; I'll be happy to run some numbers for you. A final bobweight the same or LOWER than the stock 318 bobweight is where you want to be; a lower bobweight will make sure you take off weight from the crank, not add it on, and that is the low cost way to go. And with the KB flat tops, all of a sudden, you will have a much better compression ratio, which will make this engine's low RPM operation a lot torquier.... which is good for a cruiser with attitude.....

FWIW, OP, the general concensus is that the cast cranks are fine for your level of engine. So don't fret too much over that IMHO... though it would be nice to stumble across a forged 318 truck crank. My son's 340 ended up with a 273 cast crank and it is (supposedly) built to breath to the upper 300's HP range.