Have you considered welding a nut to the fill plug so you can get it off? Then replacing with one of the low profile hex style plugs found here: https://www.marlincrawler.com/transfer-case/parts-upgrade/tc-hilux/low-profile-magnetic-drain-plug-alum-gasket

To answer your question it is 3.2 quarts capacity, however most guys just put in a full 3 quarts. Make sure you let it drain for a period of at least 2-3 hours if not overnight to get all your old fluid out before putting in your new fluid.