Confused--gm floor beds underside

[72cheyenneman]

I'm restoring my grandmother's '72 Cheyenne 10, and have a question about paint. Should the underside of the the truck bed be the color of the existing exterior color? Also, underneath the wheel tubs? I'm trying to keep it with factory specs and can't find any literature on it. In addition, if it was painted the color of the existing exterior, did GM apply an undercoat on the bottom of the bed floor? I noticed that the cab has an undercoat on the bottom of the floor pan.

Any help would be appreciated.