There is a step drill which is made for sheet metal (different than what Dennis found). It has closer increments of steps and does not catch the sheet metal and destroying the part. They are somewhere around $20-$30 range. Electricians use them for drilling holes in panel boxes. They are nice.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission