If you routinely are taking 8mg of dilaudid I'd say you have nothing to worry about with 20mg of methadone. It may make you quite high for a long time (4-8 hrs approximately) but as long as you don't take any other respiratory depressants (alcohol, benzos, more opiates!) you should be fine.
IME worrying about overdose can cause panic attacks so just relax, grab a big bottle of water and enjoy the euphoria for a little while...
And I should add, I take methadone 20mg every 8 hours for last 18 months, so I am not talking out of my ass (although that would be kinda cool if physically possible)