Structured data: mixing JSON with Microdata causes errors

I'm trying to mark up an article with structured data. For this particular article I've found it a lot cleaner and simpler to use JSON rather than Microdata. In Google's Structured Data Testing Tool this script works fine with no errors:

<script type="application/ld+json">
    {
      "@context": "http://schema.org",
      "@type": "Article",
      "mainEntityOfPage": {
         "@type": "WebPage",
         "@id": "https://my_website/my_article.php"
      },
      "author": "My name",
      "name": "My article",
      "headline": "My article",
      "datePublished": "2017-06-10",
      "dateModified": "2017-07-23",
      "image":{"@type":"ImageObject","url":"https://my_website/images/my_logo.png","height":768, "width":782},
      "publisher" : {
          "@type": "Organization", 
          "name": "My site name",
          "logo":{"@type":"ImageObject","url":"https://my_website/images/my_banner.png","height":60, "width":600}
        }
    }

However, the article body is very long so I don't want to put that in the script via articleBody. Instead I've used Microdata:

<div itemscope itemtype="http://schema.org/Article" class="col-md-8">
     <div itemprop="articleBody" class="card">
          <!-- Rest of my HTML -->

As soon as I do this the testing tool gives an error for every single item in my JSON, saying those fields are required.

Is there a way to do this without duplicating 3,000 lines of content into the JSON script articleBody?