I tried posting the whole thing.it kept making me cut back the content until I got to 300 words.
Probably operator error
If you put in that it’s an excerpt, then it will limit the amount of words you can use.
If it’s not labeled, “Excerpt” they you can usually post the whole thing.
However, there are restrictions on which sites allow the posting of the entire article, so if in doubt, excerpt it. That way it’s less likely to run a foul of posting guidelines which are hopefully going to protect FR from litigation for copyright infringement.