Just my opinion, but I think you have the brackets in the right places. The rear mounts with two bolts, corresponding to the existing frame bolt holes, and the front bracket only one bolt. Also corresponding to what I can see from your pictures. Should be easy enough to add some flat washers to the bolt to fill the gap and secure the bracket to the frame without contorting it out of shape.
I think you want it butted up against the frame rail edges when properly tightened, but I would leave final tightening until you have the tank in place as you want the bracket correctly aligned with the tank and not biting into it when securing the support strap around the tank.
