I would consider how the cars were assembled, and in what sequence. The main body would have been through "the dip" for anti-corrosion sealing/protection first. Then any spray primer and body color would come later. As mentioned, the cowl area would be body color, as would be the front end sheet metal (painted "unassembled" to the body, or at least "cut-in" before the final color coats.
I'll admit I've not investigated what's correct, as almost all of the cars I've been around were factory undercoat cars. From a cost perspective, I can't really see any significant body color being on the underbody areas, especially not to the extend as the exterior body panel areas. Probably more in the "overspray" orientation, I suspect. None of which would be evident if the car also has the factory undercoat.
Of course, the stub frame would be satiny black. There used to be a vendor at Mopar Nats that had the allegedly correct spray undercoat. But their samples looked more like "cow pies"-texture than what was on our Chryslers ('66 & '72). So if it's "thick" and has "the texture", it'll probably be close enough.
Just some thoughts,
CBODY67