Hi Rob,
One area that can be reduced a bit is the top of the webs between the cylinder sleeves. This has the added benefit of opening up the water jacket for better flow.
Aside from that, I'd mostly just clean up the casting and remove minimal amounts from the block itself. I doubt we are talking 20-30 lbs. overall, though. Afterward, sealing with Glyptal might be advisable.
I know you are talking about the block, but an aluminum cyl. head would likely be the biggest single improvement, although very pricey. Probably 20 lbs. could be saved there, and it's high up so it effects the car's center of gravity more than most pieces. Also, alu water pump housings are available, not sure how much weight savings that would amount to, but should also give slight improvement in cooling.
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