Well, believe it or not, the lightest SBC aluminum block is actually a Chevrolet Performance One. Weighing in at 90 lbs, here it is:Here's something to think about, one of those blocks is made by a Chev with a billion dollar research department and 100 years of practice - hell, they invented the shallow block V8 and have made millions of them and all quite successful, personally I'll take the extra 12 lbs for that safety margin over a small company that's made a couple of dozen of them.
As for the small companies in the business, MANY, I say MANY of the major breakthroughs in engine technology, be it in terms of lightness, eficiency, durability, you name it, come from small businesses. Sometimes MASS production leaves no room for those achievements. And you can be sure that GM sees, hears and copies a lot from those companies in order to improve its products.
Aftermarket businesses like Dart, Donovan, BMP, Callies and many others are living proofs that aftermarket can, sometimes, be much better than stock/oficial.