IC, Even though you have it under control, you may want to fiberglass the bottom of the aluminum peice back up. I dont know whats holding it in now, but there may be a chance of it falling out (Vibration,etc- theres nothing holding it "up"), I don't know about Carpys stuff, but most Vintage 'glass, has very little 'glass on the top of the 'glassed in petcock mounting plate and gas may easily leak around the mounting plate.
For future reference,
Use a smaller/tiny drill bit to find the holes first, then drill them out to the needed size.