at least on a panel, you don't have the side windows leaking water inside! Yes, look for rust all along the bottom of both sides as well as bottom of doors. Panel is not much different than a pickup for rust spots, just like a pickup with a super long cab!
Look at the floor in the rear, I believe it is plywood with hard rubber on top of it. (My 59 Suburban was). Look right in the back, center, inside the doors (assuming barn doors) and seeif it has been leaking there. If so, it should be addressed.
