This may not be helpful as I don't have experience with the early 70s models, but in 68-69 for example, the undersides were primered a grayish color. The body color was feathered in on each side a bit beyond the rocker panels, but the majority of the underside was primered.
This is how I did my 69 roadrunner, similar to factory.
More examples of how Lynch Road '69 B=bodies were underside color over sprayed with the bottom center dip gray (primer?) corrosion bath left unpainted body color.
This is a low mileage survivor Super Bee, you can see the Bahama Yellow on the outside and the gray dip still inboard.
There's more examples but can't get to them right now. I'd do a gray epoxy paint, not primer, then over spray it on the out side in body color if I was looking for O.E. but really I think it's best and less time and prep to just do the bottom body color and be done with it.
