Standard bore for a 270 is 3.781". 302 is 4.00" Both the 270 and the 302 have a 4" stroke. A 270 MIGHT accept a standard 302 piston (a 0.219" overbore) (resulting in a VERY thin cylinder wall, but putting oversize pistons for a 302 into a 270 block would be VERY risky. A .080" overbore in a 270 would end up with 281 cubic inches. That would be a 3.861" bore size.

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