You could always cut the piece out from underneath, joddle the edges down and fit the new piece from below, but weld from above facing down on the joints. Doing that way you might not have to remove the mounting flange, just drill a couple of holes through and plug weld, hard to say without seeing how much has been cut out.
Isn't the flat bit covered anyway, so the cosmetic side of it doesn't really matter? On my hatch (and the Firenza, for that matter) it's got a thin sheet of rubber covering that horizontal section.
I get the alarm thing, you have to consider scrotes reaching up though it's not so bad with OHC cars as there isn't much room. I had my alarm horns mounted on the bonnet hinge for just that reason.