"Look at the floor in the rear, I believe it is plywood with hard rubber on top of it. (My 59 Suburban was). "
Rear floors were exposed wood in 47-55 panel & canopy trucks (a 1952 panel in this post had plywood - see below).
http://www.1954advance-design.com/A-D_LCVA/LCVA-6-50.html" . . . the [1950] floor boards of panel and canopy express models were changed to plywood construction In the new construction a single panel of wood and five skid strips replaced thirteen floor boards and twelve skid strips. The new floor was positively sealed against dust leakage and the plywood made the floor 25 to 30% stronger than the former construction."
47-55st Suburban floor boards were high-density plywood with a ribbed linoleum "mat" glued to the one large piece of plywood.
