With the rear crossmember removed, the trunk floor will slip in below the tail panel. the tailpanel is attached to the trunkfloor anyway, so only a few more spotwelds need to be seperated at either end to allow the trunk floor to pass through.
Don't forget to weld on the gas tank strap supports first!

Note that on this car, the trunk extensions were left in place too. The AMD trunk floor fit was perfect.

a tip about plug welding: I like to predrill my panel, then screw it down snug. I then remove a few screws, drill into the rail flanges slightly to ensure the weld gets ideal penetration for a strong plugweld. Practice will teach you how much to drill. If you just try to plug weld directly to the rail flange you may not get adequate penetration and have weak welds. This has the added benefit of creating nice nearly flush plug welds that need minimal grinding.
Have a look from below often while welding to see if the weld has created a nice little button that shows it has penetrated well.