I had my 65 389 bored from 4.0625 to 4.120. I did some research and found that these old blocks can be bored pretty far. I felt comfortable going .0575 with mine, as I asked a very reputable Pontiac builder before attempting it. I used forged sealed power 400 pistons .5140 rods.
When in doubt...get it tested.
Fact is stranger than Fiction