I'm sure a resto specialist can elaborate on the subject, but as evidenced on my own 68 RR and other MoPars I have seen, the prep was done something like this:
The bare body shell was dipped up to about the middle of the door posts in some charcoal gray colored rust preventative stuff. After that everything was sprayed with zinc chromate primer - green for my 68 and red oxide on my 69 (maybe because the 69 is a red car?). The cars were painted with all the panels installed and aligned, which is why all the bolts and hinges get painted, then the engine with the suspension in installed from the bottom.
I have seen the entire bottom of cars painted body color, and while it looks nice, it is not correct. Only overspray in a line of sight pattern was deposited on the undercarriage. Again, looking at my 68, I see the charcoal gray primer is smooth in texture and somewhat resembles the old washing machine drum color but without the large white speckles.
I believe the wheel wells were heavily oversprayed with body color and later covered up with undercoating - though not very well - so the body color was visible in some spots. The guy that painted my 69 knows squat about MoPars and I had to give pointers and there are still things I would have done differently - like not paint the car with the K frame and suspension installed. This resulted in me painting the engine compartment after the fact, and though it turned out good, it would have been much easier and more correct to do it all at once. I will do the 68 myself in the way I just described.
One interesting fact told to me by my painter, who had a buddy that worked as a painter at the Ford plant in Milpitas, CA (DSO 72 San Jose plant and currently the great mall). It was said that painters were paired up for technique and spray patterns with one on one side of the car and one on the other and they had a minute and a half to paint a car, all while the car was moving. The guns didn't have cups; just hoses with valves to switch the color. If they were painting red cars and suddenly a green one was called out, they would just switch the valve and spray in the corner until it came out green.
Anyway, hope this helps.