I just looked at an older 9" that had a drain plug and drilled and tapped the housing on the bottom. Then I went to Ford and got a new plug to screw in. I had a 65 Galaxie with the 9" and it came with the drain plug so I looked at how it was in and that was what I used as my guide. I do believe the housing in the Cobra is a little thicker than the one in the 65 Galaxie was, but am not sure as that was a long time ago.
Ron