I was missing the requestLimits
element as in:
<system.webServer>
<security>
<requestFiltering>
<!-- This is the value in **bytes** -->
<!-- 524288000 bytes = 500MB -->
<!-- The maximum for this is 4,294,967,295 (3.99GB) - http://stackoverflow.com/questions/4022434/-->
<requestLimits maxAllowedContentLength="524288000" />
</requestFiltering>
</security>
</system.webServer>
Another useful answer to a very similar problem: Tridion 2011 SP1 HTTPS Publishing throwing 'Post size exceeded allowed limits'Tridion 2011 SP1 HTTPS Publishing throwing 'Post size exceeded allowed limits'