I took some measurements on my 67 coupe for you. I measured the inner rockers from under the car since the transmission tunnel gets in the way of measuring inside the car.

At the front, I measured 47 1/8" just behind the front floor pan seam. At the rear, I also measured 47 1/8" just behind the rear foot well. The pinch weld seam is kind of wavy, so it could vary by 1/8" or so depending on exactly where I took the measurement. In the rear picture, my passenger floor pan has been removed (rockers are original).

Edit: As best I can tell, my door striker measurement is 61 11/16". The door frames stick out in front of the strikers and make it hard to line up the tape measure. The strikers are designed to be adjusted in/out and up/down, so expect some variation in this measurement between cars.

Pete