How to find out the highest converting posts for content upgrades?

This question is primarily to people who use HubSpot, but if someone knows a work around through another platform please let me know!

Tools I'm talking about:
- Hubspot
- Wordpress
- Google Analytics

I've been analyzing the content on my company's blog (hosted on WP), and I've hit a roadblock in terms of optimization. I've been through Hubspot and Google Analytics and can't find the data I want:

How to find out the highest converting posts for content upgrades?

For example, I've explored Hubspot and the most I can find is information on the CTAs such as "clicks" from each post, but no submission or whether they're a new contact.

I can get a list of who has downloaded a piece of content from HubSpot, and whether they're new or not, but not which post they have converted from.

So the real question here is: how can you get a mass list to analyze which your most successful post for converting on content upgrades?

Or to put it in a scenario: You launch an ebook, you run 5 posts on your blog to support that content and your aim is to get new and current readers to download your new ebook.

How can you get a list of conversions which point to the most successful post that's converting readers?

Thanks for all the help!