I've been using the regular shoulder strap too and frankly, that seat isn't going anywhere. Very snug fit, plus the belt clip (installed within an inch of the buckle) keeps the belt from advanicing.
If we have a second child, I think the stang will no longer be used for the then two little ones. No way I can fit a car seat behind my seat, as I'm tall and have to keep the driver's seat all the way back. The only way to fit the one current seat on the passenger side is to adjust the front passenger seat almost all the way forward. If you're over 5'6", there's no way you'd fit in my passenger seat with the child seat behind you.
Back to topic: Not sure what you're car seat's instructions recommend, but mine stated that the shoulder belt and belt clip were acceptable/legal.