Thank you blcos, I appreciate that.
I didn't know what level of tech/fabricator you are, and I now see you are very competent.
My two-fold proposed solution is,
A) IF, the Support Plate you have for the motherboard you have on hand, is too small in area to just rivet it to the existing metal 'Ribcage' metal frame work,
1) Cut the inner part of the existing Support Plate/Framework, so that there is a large rectangular hole in the center, leaving a 'Frame' around the edge.
The 'frame' being approximately 1 inch wide, or whatever is compatible.
(Would resemble a picture frame when done)
Leaving enough area for the new Support Plate to rivet to it.
A minor, (Major?) problem is that the existing Support Plate would have to be removed from the 'Ribcage' frame work, in order to cut a rectangular hole in the center.
(Unless you have a large hole made, and use a metal nibbler cutter.
The rivets would be drilled out, with pop rivets to reinstall.
The brass standoff's, or plastic spacers would have to be the right height, in order for the I/O ports on the motherboard to line up with the holes in the I/O Shield.
Or modifications made to them, or the I/O Shield.
The I/O Shield not only keeps dust out, and helps form a tighter seal for air flow inside the computer case, but also is a Radio Frequency interference shield.
B) Proposal 2 was to use the existing Support Plate, and mount the new Support Plate on top, with modifications to the 'Bumps' on the old Support Plate, and to the I/O Shield, and spacers used.
(Plastic spacers, or metal brass standoff's)
As you stated, flatten the Bumps down
The I/O Shield may have to have one side reduced, (Outside case edge), so that the I/O ports will line up with the holes in the I/O Shield.
The extensions, (Spacers or Standoff's), may have to be reduced in height, or substituted with a compatible item that will work, so that the I/O ports line up with the holes in the I/O Shield.
Whatever line of modification you use, I suggest that a gap be created in-between the Support Plate, and Motherboard for air flow, and also to keep the solder joints on the bottom of the motherboard, from touching the Support Plate.
If possible I would like to know how this comes out.