Why get a fresh block? Have you run insane amounts of nitrous through your old engine? Have you went out of your way to create a knock/detonation situation? Run sand through the intake?
If a 331 uses a .030 over piston then I would consider keeping the old block. It would be better anyway to tear the engine down and have the block checked out and see wahts going on before you order your kit.
One of the benefits of a used block, is the thermal cycling it has gone through. A new block has not been subjected to the this heat cycling so it still isn't as dimensionally stable as your old block. Where this really counts is when you take the time to "blueprint" the engine. All that money and time will be wasted if things like the lifter bores travel a bit off from the "blueprint" specification, or a bore jacks out of round causing some loss in cylinder sealing, etc. This thermal cycling could have also orientated the block material's grain structure making it a wee bit stronger too, but I have never heard this put forth as solid?
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