Many vintage rigs of that era had the antenna mounted by the front window. My ’67 also had the stub of a cable in the rear and I assume it ran through the ribs under the windows to the front. I never found evidence of an antenna every being mounted however.
I had to replace the roof-mounted antenna in the ’72. Just like the power lines it was run through plastic insulators in the ribs and was very stiff and brittle. The old insulators pulled out while trying to feed through the new coax and would bind up in the next rib or bend and get caught on the insulation. A royal PITA that turned into an all weekend job.
The old antenna, if you have one, should work well enough if you’re planning on keeping the vintage look.
See if you can determine in which direction the grommets were installed. Make a thin splice between the new and old cables and run the new line “with the grain” if possible. Wire in your transformer and you're good to go.