Just passing along some information here on the trunk pan and frame-rail assemblies you can purchase from real deal steel. I wrote them asking what was the installation procedure and will paste the reply.
Daniel, It is bet if the main floor is already installed. That way, the forward frame rails and forward edge of the trunk floor have a place to land. From there, it just needs to be square. If you need a measurement from a point on the floor to a point on the trunk floor assembly, we can help with that. Or, you can purchase the entire assembly:
