1. Admittedly, we’re rather new to complete engine rebuilding. Replacement engines and top end kits have been more of our thing over the years, so we tried to do a little Web research before purchasing a bare block to start with. We ended up with a 3970010 block also cast with the numbers “010, 020, and 509,” which seems to make it a sought-after core. There was too much contradiction online for us to tell you anything for certain, but it seems ours is an earlier block that may (or may not) have a higher nickel content and thicker main bearing webs. We’ve also been told the cylinder walls are a little thicker on earlier blocks.