need a little help with glove box

[undertaker72]

cab some of you more experienced builders shed a little light , what is the best way to install a new replacement glovebox ( the cardboard unit), its like trying to fit a tuna can in a pepsi can , do I need to remove heater core , I sure didn't want that , the delapedated one came right out , from the mouth the new one dosent quite want to , any help would be grateful , this is a ac cab 1972 thanks again kevin