Wow, there is a lot going on here.
So, I assume the order of application on this car was:
1) Gray primer on undercarriage
2) Gray primer in wheel wells
3) Sound deadener on undercarriage after axle and brake hardware but before driveshaft and exhaust
4) Sound deadener in wheel well after installation of suspension and axle
5) Body color with overspray on undercarriage
6) Engine compartment black (not discussed here)
Uh ... no can't agree with much of that based on how the cars were typically assembled and its not like they could pick up sections of the line and move them around the factory to apply just to your car or even a group of cars IMHO IF we need to follow this line of discussion I think we'll likely break it away into a new thread
1) Gray primer on undercarriage- Will leave that alone since that is the basis of the original question
2) Gray primer in wheel wells - As mentioned a different gray, a gray primer surfacer was typically applied to the exterior body to fill the small voids and imperfections left in the red oxide application
3) Sound deadener on undercarriage after axle and brake hardware but before driveshaft and exhaust - No the sound deadener applied to the floor was most likely applied at the same station or one before or after (given the needs being different for spraying verses installation of components) when the front wheel well sound deadener was applied
But yes the sound deadener (if you have a Mustang model that was done that way) was typically done before exhaust and maybe driveline. Still working out that last detail as it may have been wrapped, shield or other wise protected from any application. Subject for another thread
4) Sound deadener in wheel well after installation of suspension and axle - The suspension and axles (and just a ton of stuff) was installed in the second part of the assembly and the rear wheel wells sound deadener was applied in the first part of assembly before body color was applied
5) Body color with overspray on undercarriage - in the trunk in the rear wheel wells and so on
6) Engine compartment black (not discussed here) - After body paint application
Though it's for 70 Dearborn you might want to take a peek at the article in the Library here related to undercarriage painting. Might answer a number of question in general
I need to apply the sound deadener on the undercarriage since my car is a Mach 1?
As mentioned above - yes Mach I's, Grandes and some other models had an application of sound deadener applied to the floor pans,frame and things attached at the time
Now back to the discussion of the gray floor possibility