Just had this same problem last night! On my 73 Cuda. The flat part goes down....(I called metro to verify) , I fit it and glued it on. Went to shut the lid ... No way was it fitting!!!
Bulged the lid up and put a huge amount of pressure. I ended up having to rip it off, and i just ordered a eariler style from columbia parts on ebay.

What i dont under stand is what is so different abount a late 73-74 trunk lid/ gutter that makes that huge weatherstrip even work?