65 and 66 389 have thin wall cast blocks, plus the connecting rods were strengthened in 1967 over 1966 by adding a rib on the big end next to the rod bolt.
Then there's what Bart said about the heads, and valvetrain being improved also. Unless you needed to have a 389 for class rules, I would go for a 400, hands down.
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Brad Yost
1973 T/A (SOLD)
2005 GTO
1984 Grand Prix
100% Pontiacs in my driveway!!! What's in your driveway?
If you don't take some of the RACETRACK home with you, Ya got cheated 
  
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