I'm looking for suggestions for a hollow-wall anchor sytem.

I'm building a set of wall-mounted bookcases. These will be quite heavy when
fully loaded.

I plan to use a French cleat system to hang the cases, but need suitable
fasteners to attach the wall-side cleat to the wall, which is drywall over
steel studs, 16" oc.

The wall cleat is a strip of 3/4" plywood, 9" wide, with a 45 degree bevel
on the top edge. I plan to fasten this to the wall with some kind of anchor
that goes through the flanges of the studs, grabbing them from behind. I'll
use two rows of anchors on this cleat, top and bottom.

So far the default choice are the standard toggle bolts with the
spring-loaded wings. My only objection with these is that the hole drilled
in the wall is much wider than the diameter of the bolt, so it doesn't seem
like there's a really good bearing surface for the bolt to resist the
downward pull of the cabinets.

Does anybody have any suggestions?

Thanks,

Chris.