The castings started getting lighter in the mid to late 70's and the castings more porus when they started making them in Mexico.
As far as I'm concerned from about 75 through the mid-80's were the "dark years" and I would avoid these castings if possible. I think the late 80's and 90's castings get a little better due to improved QC but I wouldn't hang my hat on that when building a serious high perf engine.
My next engine will be based on an aftermarket block for sure. These blocks are only about $1000 more than the cost of buying an old core and getting it machined.
E85 racer and E85 carb builder
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68 Camaro, E85 powered 427" small block. 9.96 @ 133 MPH, 1.319 sixty foot on motor. 5.92 eighth @ 116 with a 1.42 sixty breaking beams with back tire on the bottle