PP has a 5 yr expiration..sorry. If you were talking like a few days, even weeks past the expiration..but MONTHS? just too big a risk, and not worth it. It's really too bad you have a PV, all the pother infant seats on the market have a 6 yr expiration.
however, you can put a rolled up receiving blanket on either side of the baby to keep the baby planted firmly in the middle of the seat, even in a large convertible seat.
here is a pic of what i am talking about. It is a graco convertible, with a 1 week old newborn in ithttp://i163.photobucket.com/albums/t...0_182125AA.jpg
and like a pp said, in a correctly snug 5-pt harness, it really wont matter how thin or small your child is. Even though you migth *feel* better about your child have soft cushy padding around them, in a collsion, its the harness taht is going to keep your child safe. And regardless, as has been pointed out, lots of carseats have special infant inserts these days, which will "cradle" your infant even if they are quite small.