There's a thing that happens when casting a block called "Core shift". It's when the cores don't sit centrally in relation to the cylinder bore and can be a problem when reboring a motor. Some 308's had no issues, some went through the wall at 30 thou. A sleeve is an option if you do go through, but it needs to be done right. I've seen some bad ones that leaked water and had to be tossed.
Having said that, yours may be perfectly OK and you won't know until you try, so give it a go.
Early blocks do seem to be more reliable, but there were still issues with some, so it's not that they were thicker, just that they didn't have so many with Core Shift. Thicker cylinder walls means narrower water jackets and therefore higher temperatures, so that's not a good idea and that's why it wasn't done. Holden didn't give a crap about whether a block was able to be rebored, only that they worked properly while under warranty.
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