leonphelps,
Looking at the photos, I would say that you do have your work cut out for you.
In addition to the rust-out condition of the floor I also see rust through in the cab vent. This would lead me to think that your cowl was full of leaves (or something) such that it could not drain. Please make sure that you evaluate all of the cab for rust and rust-out conditions. Rust is like an iceberg, what you see is just the surface and the bulk of it is what you can't easily see.
If you still feel that fixing is better than a donor cab, I would suggest the rocker panel (#20) shown on this link:
http://www.lmctruck.com/icatalog/cbe/full.aspx?Page=9
verses patching with the inner/outers. You may want to get these first to determine what other specific parts you need (I did not use these so I don't know if they come with the innner parts attached or if you would want to use the parts that come with the floors.
You may not need all of the floor section but you may not be able to make the intricate bends needed. Plus what you don't use you will give you extra metal to work with.
On this link:
http://www.lmctruck.com/icatalog/cbe/full.aspx?Page=11
For the floors you could get items #2/3 and #6. The other parts it looks like you will also need #1 and #7.
Take this with a grain of salt. I do not do bodywork for a living so mine is only an opinion and may not be right for you.