When you use the main bearing spacers, you use the 340 crank to make it a 3.31 stroke. Sell the 360 crank, and sell your .040 pistons for the 340. Bore the 360 to 4.040, stock bore for a 340 and buy the pistons in the link below to use in the freshly bored 360 block. The biggest expense would be the bearing spacers.
[FOR SALE] - Used std bore pistons - 340 | For A Bodies Only Mopar Forum
The other most frugal option would be to sell the 340 rotating assembly and buy a set of KB-107's or similar pistons for the 360 and roll with that. I have had dozens of 340's over the years, but I have a mildly built, well-mannered 360 in my 71 Challenger conv. It's a ride around and enjoy it type of car that is plenty fast enough for what it is. My conv has 340 Four Barrel badges on the hood and 99% of the people can't tell the difference by just looking at it. Good luck to you. You've been put in a tough spot with the motor, but just make the best of it for you.
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