I found that it was a bear of a job and required patience. I recently installed a headliner and new rubber in my 59 GMC.
Get some water and some dish detergent soap and keep the metal and rubber wet with the soapy compound. Cover your seat with a cover if it is in place or, remove it first.
I made some small blunt pine wedges and placed the wedges about 5-12 inches ahead of the rubber that I was working onto the roof and metal bead. I then used another set and placed those on top of those wedges and in effect used the second wedge to pry apart the sheet metal from the roof enough to slip the rubber into the formed metal bead etc. I used this process all around the perimeter.
It was much easier this way to use the double wedge process to hold the metal apart enough and use my two hands to slip the rubber onto the metal and then seat it.
The lead hand positioned it, the second hand then forced it into the bead. Sometimes I had to use a small block of hardwood (I round it over to prevent cutting the rubber) and with a no-mar mallet and block drove the rubber in place...especially around the corners. It tended to pull away from the corners and each corner required several attempts by pulling it off and starting over at the corners. The rounded block of wood really helped drive the rubber into those corners and you cannot go further until it is seated into those croners.
After awhile it was done and the process got easier as I figured it. Don't be timid pushing those wedges in to open the gap. I found that the metal was tough enough and it did not damage the roof because the wedges were blunt enough.
They were (about) 1" wide but maybe wider ones would work even better. The hardwood block was (about) 2 inches wide and I rounded it over on the sander to mimic the corners (but with less radius).
HTH. Good luck.
tom
