Have a look at this complete section 1969 only panel... it is designed to fix rusty floors or damaged entire floor sections as shown in the picture...
This is a multiple parts.. assembled panel.. made up already & includes the braces under the pan as well as things like the bumper bracket reinforcments etc... it NOT JUST a wider pan.
There is also much cheaper sections available that just does the main trunk floor area's if that is what your looking for or need.