1960-1000 : Approximately 1" hole. The wires have a bushing or plug in this hole. It is late and I didn't feel like taking the wiring apart in the dark.
1961-1500 : Again, wiring goes through either a rubber bushing moulded to the wires, or it is a plug (male and female) in the hole.
1961-1500 : This one has the cover plate over it. Makes the floor look "nicer" and hides the wiring.
1962-1000 : The wires go through the firewall in 1962. All that is left is the depression in the floor
1963-2500 : On this truck we put the switch for the 3 tanks in the depression.
1963-4000 : Just the depression
1964-2500 : Just rust and the depression
1966-1000 : Just the depression
So, it looks like only 1960 and 1961 had the wiring for the taillights go out through the floor. In 1962 through 1966 it went out through the firewall but the floor pan still has the depression for the wiring.