Oh yes, call the vet. Just to calm your nerves if nothing else and all is well. That way IF something is wrong, you may catch it, and if nothing is wrong, he may be able to give a rough estimate as to when.
Hey! I'm the one whos mare just had the foal that was overdue.
She was on day 368...and she was a maiden mare as well.
If I were you I woudn't panic until day 380 +...It's really not that rare for a mare to go over the 342 days. That is just a guidline not set in stone. I would give her 10 more days and see what happens.