1971 340 Build - Choosing piston rings dilemma

Glad to help, even though I'm sure you're disappointed to have to hunt more pistons. KB243's are what I have bought for a future 340 build. You could do like @Jadaharabi suggested and get another crank. If you can find one in your area, you can get a 360 LA crank, and have the mains turned down to 318/340 size....have it balanced and use it to make your 340 a 372ish CI motor. I asked my machinist about doing this with a 360 crank in a 318, and he said sure, no problem. He said he would also turn the counterweights down to make it internal balance. He built a 360 a while back for a 4WD truck, and did the balance like that so the guy didn't have to buy a new flywheel. I watched it make 380 HP on the dyno, so I wouldn't be afraid of doing it. Just for kicks, and to maybe show the difference....take one of your KB 107's and one of your stock 340 pistons, and slide them onto a wrist pin side by side, and take a pic to show the difference in pin to crown height of the 2 pistons. It would be interesting to see the difference. Sorry you have to get new pistons or fool with the crank, but I'm glad it was caught now instead of later.