A bit different as mentioned but hoped it would provide some help. OP can basically trace the line from gas sender forward an inspect the flex lines for this and the vapor return line
Ugh,
If there’s hell on earth, cleaning the underside of a 50 year old car most certainly is it. A dirty, miserable endeavor.
Making good progress - cleaned up to the rear floors today.
Have dust and grime in places unmentionable in polite company. The Cougar Forum too. 
On a positive note, this car has the nicest, most rust free undercarriage I’ve ever seen.
Nice work! What will you recoat with? I know how bad that job is after tackling it on my car last summer. Mine had been undercoated when new (factory fresh paint underneath) and was really hanging on tight. So I gave up long before you did. Settled for scraping off only what would come off, cleaning and wire brushing for adhesion, and then lightly recoating with 3M rubberized undercoating.
I’m leaving as much of the factory paint as possible. It’s well adhered and should be good to topcoat over. Plan to prime and repaint the floor in body color, replicating the factory patterns. May re-apply undercoat in certain places (panel seams, etc)
Probably another couple of days to get the underbody clean however. Not looking forward to it. 
What color did you find the floors were painted with for this particular car? Looks to be one of the dark gray mixtures is that correct?
Besides the body color overspray along the outer areas of the floors, further in around the rear end area and the rear wheel wells don’t forget the pinch weld black out and the overspray pattern it produced along the same area behind/inward of the rockers and quarter panel drops
Car is a code C dark green poly, and the entire underbody from trunk to trans tunnel is body color. Not just misted green from overspray, but painted that way.
The insides of the rear wheel houses are red oxide and body color with undercoating over. The trunk crossmembers and the sides of the rear frame rails are a grayish color.
Never seen that much body color on the underside of any of my Fords…
That’s the stuff of nightmares! One of the scariest moments is hooking up the battery for the first time. I feel your pain! Good luck with your project!
Thanks Sounds like the often found gray batch color. Red oxide was also used during some of the production period Some times the batch color is similar to the exterior and owners assume its all body color while other times the two are very different. Red body with a green floor for example. Not sure how body color could have fully coated the floor given the process and where the painters stood and walked while painting the exteriors at Dearborn but good luck with your project.
I’ve got an old Ford ad from 1969 showing how they dipped the lower half of the body in electrically charged red oxide primer. There is a Lincoln Mark III getting dipped in the photo. Is this how red oxide got applied on Mustang/Cougars too, or was the dip only done on Lincolns and full sized models?
Yes one of the challenges in figuring out how things were done since different lines (building different models) did things differently.
Guess your asking about the undercarriage the rest of the preparation of the uni-body is a different process with a couple of different primer/sealer and primer applications
Mustangs and Cougars were built cheaper so they didn’t get some of the same treatment top of the line models did. For classic Mustangs and Cougars the floors were painted by passing the body over jets mounted below the moving body that were tripped to spray when the jets were about where the bottom of the firewall bends rearward. At a later station a painter in a pit would spray and cover the bottom of the forward frame and other related panels with a spray gun.
So no dipping for the unibodys for our cars though engineers kept the floor panel design features the same as those that were dipped. Guess it helped with the reinforcement of those thin panels.
Hope I explained it clearly
I’ll take some photos. It was a surprise to see how thoroughly the underbody is painted in the body color. I expected to see traces of body color as overspray from the paint process, but the bottom of the car(from the trans hump to trunk floor) is almost entirely and methodically painted in code C dark green. The sides of the frame rails look like galvanized coating, with body color only on the bottom of the rails.
Stay tuned for pictures…
Jeff and others,
Here are some underbody pictures. Lighting isn’t ideal, but the color is not gray. It is absolutely, 100% positively Code C Dark Green Poly. It covers the entire transmission/driveshaft tunnel, rear floors, convertible braces, trunk floor, frame rails.
The crossmember for the staggered shocks is however a very dark gray on top of galvanized.
There is ZERO question that someone painted the undercarriage after the car was built. This undercarriage is untouched since it left Dearborn.
Very strange.
Of course back in 1969 before poly, Code C was known as Dark Ivy Green Metallic. But that’s definitely a lot of it that was applied under your car!
It sure is.
Getting ready to spray PPG red oxide on the chassis. Prepping the undercarriage was no fun, but it will be nice to have the underside clean and reasonably correct.
Will post pictures when finished with the painting.
Next steps:
- Cleaning and reinstalling fuel, vent, brake lines.
- Cleaning and repainting the 9" differential housing
- Installing a 31 spline, 3.70 geared center section with new 31 spline axles
- Cleaning, detailing rear springs
Don’t know if its been asked or I just missed it, when was your car completed at Dearborn?
Some examples I had handy
Top one shows a floor color similar to what your reporting but in this case the exterior color is a lighter color that contrasts with the floor color. That is from a very early 71
Light gray versions
Red Oxide primer sealer period
Do have some dray gray with blue or greenish tints (very typical on allot of 64-73 Dearborn cars) I believe also some where
That funny! The lower one is from my 71 resurrection’s thread.
You will find many more pictures here;
https://cccforum.discoursehosting.net/t/wow-up-beat-cloth-and-vinyl-100-original-paint-71-xr7/5160/1
I had forgotten that the area above the gas tank was body color. The rest forward to the fire wall was slop gray.
Nice job on the underbody. speaking from experience it’s the worst… my 71 was rustproofed 2 or 3 times then heavily undercoated at some point. My car probably lost 100 lbs!
What are the 2 small u shaped brackets welded to the bottom of the trunk floor near the shackles for? Mine has them too but nothing was attached to them.
Jim
Rear exhaust hangers
Can somebody post a photo of the correct fuel tank vent line routing? The hard line that runs from the tank along the transmission tunnel to the charcoal canister.
Thanks!