I replaced the plywood in my 65 suburban. It was 3/4", 2 pieces held in place with caged nuts along the parimeter and it had those countersunk washers with the off center square hole for use with carrige bolts. It was a lap joint down the middle.
I folded the aft edge of the back floor forward to get the lip off the wood and pulled each piece inboard and they came out fine. I used them as templates and used exterior grade plywood that I treated with water proofing, no problems!
