stick aluminum heads and manifold on it now you're at the same weight as a small block. And actually I don't think they're any more expensive to build maybe a little cheaper.
No offense, but I wish people would stop saying that... Heck, build the BB with a Donovan aluminum block and you'll be really light. Light in the wallet, too.
You can throw aluminum heads and manifold on the small block you already have and be light as a slant six, too.
Modified bb vs. factory sb weights? Apples and oranges. Compare them with the same mods.