well looks like sort of a heatsink.
What I would do is just mount it without the backplate, but make sure that the heatsink on top is still VERY safely and securely attached!!!
Then, before you do anything else, get a temp monitoring app and monitor your temperatures to make sure they are all safe, particularly when under load, using a stress test like furmark.
Once that is good, I think you should be ok.
I've never really seen the 8800's with these kind of backplates, so I suppose it should be ok, but make sure you run a few tests to be sure.
Dear others, please correct me if I'm wrong.
Cheerz and good luck!