Checked fit of left tank attach bracket and discovered I had made a mistake. As indicated on the next page of the instructions which I hadn't read yet, the mounting bolt holes should not have been drilled during fabrication; they should have been match-drilled to existing holes in the fuselage. The variation of hole placement was fairly close, so I thought I could match drill the undersize fuselage holes to the bracket holes. Difficult access; had to use angle drill and had numerous problems. Broke #30 stubby bit in upper hole; cleared with punch. Started drilling lower #12 hole, but bracket moved and started hole in wrong place. Re-centered; started drilling hole again and #12 bit broke loose from its threaded collar. Used bit stub in adaptor collar and managed to final-drill hole. Messy, but salvageable.