Sorry, I have no idea on price of an alloy 5.3.... never chased one or had anyone local mentioned about one to me off hand. You could look around on ebay a bit for a general idea at least perhaps here.
As for engine numbers...iron or alloy should all have the VIN number stamped in them somewhere I would say. (never looked at the 5.3 in my own truck) but they do it from the factory for all new stuff. Suppose new crate engines would be exempt obviously at least from that.