Custom post content to display in a normal post?

Solution #1 – home brew

You could

• add a metabox using add_meta_box() ^{Example on the bottom of that link}.
• In your MetaBox callback you could add a Query like this one, but for posts
• Then use the jQuery UI Autocomplete plugin that is shipped with WordPress like explained in that answer.

Solution #2 – from the super market

But you could as well use “Posts2Posts” which can be found

• in the official repo on wp.org
• or on GitHub where you’ll find extensive tutorials in the Wiki and help in the questions/issues list.