First welcome to the site - hope you find it useful
First we have no way of knowing what color your car's original floor and front frame rails were originally. But since all evidence, according to you, is gone your left to using what others from the same plant and time period have/had and go from there.
With that said it appears that your car falls into an odd, unusual or different period at Dearborn in 68 on the Mustang line. Looking at other samples reported to have been built within a week or less of your car I offer the following
Below are two examples of the blue-gray batch color that they were applying to both the floor section (firewall rearward) and the forward frame and inner fender panel section (firewall forward)
They likely started with a gray epoxy primer sealer base and added left over or old body color paint from the shift or day before. More blue cars (for example) more blue tine ...... you get the idea ;0
In this one the paint that was applied through the seat anchoring hole was protected the best for a sample
And another from the drive line tunnel
Original colors could be different than what your seeing on your screen due to camera and monitor settings
At the front of the car the painter would have applied the color to the bottom of the frame and brackets welded to frame as well as up into the front wheel well on the tire side - shock tower and surface of the inner fender panels behind the radiator support. Of course much of these last surfaces will receive some or allot of body color and or black engine compartment paint
Floor color would have normally faded out before it reached the rear cross member. Sure your already aware that on top of the floor color you'll end up with body color and pinch weld black overspray
Hope this helps
