When I ran into this I built out the wooden framing with 2x6's so it was flush with the concrete. Then ran studs down from the wooden framing and bolted them with anchors to the concrete foundation. Then threw an oversized piece of 1/2" plywood over the studs. It made a nice mounting surface and could staple NM to the plywood where necessary. The plywood was also a great place for mounting a distribution for telephone and cable lines.

Contact your inspection authority in your area. As long as you are being legal installing it and have a permit they will give you a locate and would allow or dissallow the panel's location.

Can you mount the panel upside down, or does the 67" rule apply to baranch breakers also?

Are Canadiens shorter than Americans? :-) Our NEC allows mounting up to 6'7", a whole extra foot.

You can mount the panel right side up, upside down, or sideways. In Canada, anyways.