my suggestion. Since most mobo's will come with a back peripheral plate, I would mark the size of that on the back, chop that out, and then mount the plate. I would also suggest chopping the entire back out (Minus the psu portion) and replace it with a solid piece of steel. (Might require welding.) Realistically, it's more worth it to go and find a nicer/newer/Not proprietary case and use that. The disadvantage of this one is that it was made to fit an HP mobo, not a regular style. (Notice the Mouse, and keyboard connectors are vertical, not horizontal)
And just in case you care the specs
nice, it's even complete without HDD rack mounts...

I can give you a couple suggestions, but it's going to be a lot of work.

If you intend to use that case. It looks like the expansion slot section is only riveted on, I would drill out those rivets, and cut some of the expansion ports out, then mount it into the wood.
I would leave the PS mount where it is, it's a fine location.
You'll have to get some way of mounting the HDD's in
I'd be interested to see the side in the fashion of an old wooden tv
only with the tube on the other side. (Make the tube the window.)