I'm sorry, I recall when the analytics code was first introduced for google it DID work fine to be in the <head>. But, many things have changed over the years, and both Google and Zen Cart want things done differently for it to work. Sorry for glossing over that in my previous reply.

Yes, if you're going to put it in a template file, it's okay to put it in tpl_checkout_success_default.php

Or you copy /includes/templates/your_template/common/tpl_main_page.php (or tpl_footer.php) into a NEW directory at /includes/templates/your_template/checkout_success/tpl_main_page.php (or tpl_footer.php) and put your google snippet in the bottom of that file.