I did this too mine , used some round stock though . Got major oil canning on both hood tops , tried to heat shrink 'em out ala you tube video . That did'nt work so I used bondo glass to hide my mess ups . Hope your a better welder than I am .
Not to sound like a smart arse but the reason you had the warpage in the first place was because the heat from the weld caused the metal to shrink. The heat shrinking only made it worse.
In addition, leaving the inner flange where the two hood pieces bolt together basically acts as a fulcrum. The best bet is to cut off the flange inch by inch and and then weld as you trim the flange away.
