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I've only tore down about 10 engines in my career as a enthusiast. I have never seen a lack of casting material like those. Usually it's the opposite, some extra flashing material that can be ground of to make it pretty or to help the oil flow ever so slightly better. I'm not saying they are not good cores, I'll let the more experienced chime it, but I would not consider them for souped-up build. You might ask "why" and I'd have to say because "I don't know any better". Even if they are good-to-go, perception of a problem can reduce their value.
 

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This is what I have seen on the three 400 blocks that have come through my shop (pic borrowed from the net)...

Photograph Automotive tire Tire Wheel Rim


The extra flashing is seen, above, between the opposing rear lifter holes as a lighter colored ridge. The OP's photos show holes or voids in the rear wall. After flipping through photos, on the net, of our beloved blocks, I see several with holes and voids in that area...must have been a common occurrence. I would still consider it to be a less than desirable casting. I can't begin so suggest that it won't hold up and I would certainly not throw them out.
 

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Along with cleaning up the casting flash around the lifter bosses, I have been advised by a long-time Pontiac drag racer to open up all the oil return holes in the valley. I have opened them up with a die grinder by about 1/16th inch or a bit more, but try to open them up towards the slope that the oil is running down to begin with. Then I chamfer the hole opening up slightly to eliminate the sharp edge. You can see one of them in Jim's photo...dead center in the photo. It's just one of those things that sounds good to do without any study to prove it. I sleep better for it.
 
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