I did this once on a set of doors off a '72. I just drilled out where the frames were welded to the doors through the front and back bolt holes. This broke the welds free and did not really require huge holes. I then took a dremel tool and using small cutting wheels I cut through the welds where the frame comes through the top of the doors. It took a number of discs, because they are so small but it worked. There were no rivets on the inside of mine. I was surprised how little work it actually took to break them free. The new frames slid right in and bolted right into place.
