Why do you want to take the methadone ? Are you trying to go through a maintenance type thing or what ? Or, are you just trying to get messed up ? I guess not knowing your situation I wouldn't feel right giving you 'safe' advice.
Assuming you have at least a slight tolerance to both benzos and opiates I'd say you should be alright (though benzos and methadone CAN be quite dangerous...so be warned). Make sure the kpin you took WAS a .5mg. Also, how much methadone ?