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As neat as owning a rough, un-machined ZL-1 block sounds like, I would limit it's usefulness to being a 110 LB conversation piece ONLY. By trade, I am a Machinist and there are so many major and minor operations that need to be performed on that block that you'd be looking at a HUGE investment in time and money. You are right about Tonawanda and Winters Foundries having jigs and fixtures back then to perform all these operations. You'd have to have a VERY good, Machinist set all this up again just to do ONE block and if any of these machining steps get botched or something bad happens, you could consider the block junk. Heck, I consider myself very good at what I do but even I wouldn't want to tackle something of this magnitude! One more important point is that I can't help but think that something must be wrong for this block to still be floating around for 40 having avoided machining when it was new and for the next four decades. There's a possibility that it may not have even been heat-treated yet. It may have been inspected immediately after the casting process and deemed un-machineable due to core shift or core sand or slag in the casting.
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"Bingo"................

.....Even if you found someone to machine the block.....I'm sure the cost would be much more than a "New" GM aluminum block....
Ken