See those oval holes, open them up and run some angle or 'C' section into it and bolt it down through the towbar bolt holes, you'll need to weld nuts to it basically it will replace the nut strip that is in there at the moment. Of course have a section sticking out and have this welded to the rear bar or better still going through the back of the bar and welded on the face (and ground smooth) and at the back. This takes a lot of load off of the stock mounts and will stop them cracking. The bar I made years ago carrys a 35" tyre and has done the corrugations of the Cape and numerous other tracks using this mounting method with no cracking.
Cheers
Steve