Don't worry about it. Its not going anywhere once its bolted up tight! If it did the factory shims would be all over the road. The one hole is locating it properly front to back and the oblong hole is still locationg it up and down. Weld a thick washer on the inside for added strength. I would do this on all 4 holes. Also you can weld all the seams on the new towers to stregthen them. Many of the professional Mustang builders do this on the new dynacorn builds.