1967 Pontiac went to open chamber heads and changed there valve angle. This means the valve comes down at a slightly different angle. The reliefs in the tops of your pistons are cut to the same angle. If you use a high lift cam the valve may hit piston if you use 389 heads with 400 pistons or vis virsa. If you want to use your 389 heads you probably will have to buy custom pistons like from Ross. Nobody makes 389 forged piston anymore.
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