The few 66 389 blocks I have sonic checked all had thin walls, core shift and weaker lifter bosses. Best bet would be to look for a good 400 block.
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There is no such thing as too much cam, just not enough engine.
1966 GTO - 11.31@119 (3478 lbs) in 1979
1967 Firebird - 9.73@138 (2940 lbs) in 1986
400 cu in / 295-298 cfm RAIV / Lunati Cam 278/283 - .704 lift/5.13 gears
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