It depends if you use a stock block or aftermarket, depends if the heads are darts, or maybe a vortec head. Light crank in the open motor? Too many variable to say exactly, but no doubt the crate is lighter, it has alum heads and it has the stock block.
The 604 weighs approximately 421 pounds as supplied from GM. You will have to weigh your engine with the same components to know the exact difference. There are options out there that are similar in weight but you still can't run a spoiler...
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