I don't have an answer either I'm afraid, though it may be worth me mentioning that with the Garmin Forerunner 305, there's no 'foot-pod', though this can be purchased at a later date and linked to the 305 (for treadmill running and locations with no/limited GPS signal).
I'm sure if you read through your 405 manual it would have some info on linking the unit to a HR chest strap - if it doesn't then I'd imagine it's unlikely to be possible. Alternatively, have a look through the settings on the unit
