As Bill stated, the best 390 blocks are the '60/'61 375/401 horsepower hipo blocks. These blocks are hard to find and the
oil passages are not drilled to allow the use of a hydraulic cam, so it is solid lifters only. I would suggest looking for a '65 or later 390 because the 3 bolt motor mount pattern is much easier to find mounts for than the '64 and earlier 2 bolt style blocks
--Mike