Hey, that's me! I've done a lot of online searching on this subject. The conclusion is it varied a lot depending on where the car was built and when the car was built. My 1970 Mach 1 from Dearborn had gloss black on the bottom from the factory. Some cars had orange. Some cars had "slop gray" which was a mix of whatever enamel they had leftover on the assembly line. So, exactly what shade of gray depended on the particular day.
I used a lot of SPI products on my build. I would suggest you strip the bottom and shoot on one coat of SPI epoxy; gray would be fine. Then apply all the seam sealer. Then shoot on two more coats of epoxy. That's all you need. Yeah, some guys like to put on bed liner or other stuff. I'm in the camp that if I see a car with bed liner on the bottom, I wonder what it's hiding.
My initial plans were to undercoat the entire bottom after stripping, priming, and seam sealing.
But then i spent forever cleaning off 50 years of crap I didn't want to cover it up with a bunch of undercoating.
Regardless of what I do It is likely the car will be better off than it was from the factory.
It seems like everything behind the differential got a thinner coat of paint (or nothing maybe) because everything from the differential back seemed like surface rust I was removing instead of paint.
It was bizzare. I guess it could have been red oxide but it looked more like rust to me and I didn't see anything that looked like red at all anywhere else on the underbody.
There were also a bunch of seams that were not sealed at all from the factory (there were some seams that were sealed on one side of the car but not the other, etc).
I'm glad this car was in Southern California it's entire life...
I plan on driving this car daily during good weather; but realistically if I put 10000 miles on it a year I'd be surprised.
Man i hate decisions like this.
I really should just get it done and move on to more fun stuff.