In v8 you need to do it with xpath. An example for the header:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <template id="report_header_custom" inherit_id="report.external_layout_header">
            <xpath expr="//div[@class='header']" position="replace">
                <div class ="header">
                    Your Code
                </div>
            </xpath>
        </template>
    </data>
</openerp>

The id needs to be unique, or otherwise it will override an existing one.

The inherit_id is the modul name and the id of the report, which you want to extend or change.

The rest ist just standard odoo xpath.

I hope it helps.