Spirit, although I didn't do the bracket work on the bike in my avatar, all the mounting points start with the front gas tank mounts. When your new tank is offered to the front mounts and the seat is offered to its rear mount, this will determine where the tanks rear mount and the seats front mount should be.
Once tank and seat are in place, you can then figure where the oil tank will fit in relation to the cut away in the seat for the oil tanks filler. The top mounts can then be figured followed by the bottom mounts, remember to leave room on the bottom mounts for your bottom cushioning.
Another point to note, glue a thin peice of rubber mat to the rear of the tank to protect it from small stones that might be thrown from the rear wheel.
Sam, CB750A.