Yes to what Thomas said.
What you really need to do is set this up in a Program like "Content Download" and use the Statuses such as:
Downloaded Content = Success
So whenever someone Fills Out Form, move their status to Downloaded Content.
The Page itself will also show you the conversion rate.
So when you want to look at someone's Lead Source and the Asset, you can then pull a smart list or Lead Performance Report:
-Member of Program = X with Status = "Downloaded Content"
-Lead Source=List Rental
and there you go.
I recommend creating some extra fields and tools to help track: