When you say "painted," do you mean painted body color? If so, the answer is no. As moparleo posted, the undercarriage or "underside of floor that faces the road" was dipped in a grey colored primer and then either was left as-is or got undercoated. Whether it was undercoated or not would be indicated on your broadcast sheet if you have it. Either way, the bottom of an original car is never painted body color or any other color other than the grey primer and/or undercoating.
Body color would be seen in the engine compartment with over spray visible partly down the unibody rails to a point just past the firewall. From that point back would be the dip primer and/or undercoating.
If you want to duplicate the grey dip primer, it is available from Resto-Rick Kreuziger
Restorations by Rick Kreuziger - Product and Component Restoration Detailed Information and then again, as moparleo stated, there would be some body-colored over spray on the underside floor area.