From responses over the years to different forum, it appears that 1st Gens varied from the basecoat black underneath with some light body color overspray at the rockers and trunk area to overspray so heavy that the undercarriage appears to be painted. My '68 was body color on the entire floor pan area, frame rails, trunk area, etc. with very light overspray fading to the black up in the driveshaft tunnel, lower firewall (at the turn upward) and upper rear seat recessed area.
It has been reported that the automated factory paint spraying was supplemented by a manual process in areas not hit by machines so there could be variations by the painters.