An assembly line flasher "should be" in a bracket attached to the left vent, or very close to it. If you have a copy pf the 1964 Mustang Electrical Manual (AM0003), pg 15, it's shown as item 4.
If you are a "large" driver (I'm 6 ft), consider removing the driver's seat and placing a packing blanket on the floor. It goes easier on the back that way.
Jim