As I have maintained for everyone with a 64 thru 73 Mustang, get a set of the Mustang Assembly Manuals. That being said: remove the quarter panels (you may also consider removing the sill plates); there is a 5/16-18 nut with a large washer accessed thru a hole in the sheet metal - use a 1/2 inch deep wall socket with a 3 inch extension to remove.
Jim