[lks dcvn]

I agree with 1949gmc - on his analysis of the area.

The real key to this fixing this area is patience.

If you want to do it right and not want to beat the truck with a hammer - it will be a LOT easier if the area is opened up.

Having just gone down the road that 1949gmc has on his blazer - I will say there are a few key areas that you will need to have installed the right way or everything else will be 'off'

If I had this area opened up like the first pic - I would do the following [not sure how much rust repair experience you have - so I will go from the beginning]:

1. Before you take it apart - get plenty of 'when i get back to this area photos' and order the needed patch panels - if you need a source for one/all let us know - we can assist.
2. Bolt the top to the truck - to help body alignment
3. Replace the body mounts/bolts if they are shot - since they will dictate body sag as you do this
4. Brace the body if the rust is severe enough
5. Remove rust and remove as needed
5. Bolt/tack weld in the appropriate patches in the following order
a. Part A in your picture [this will align the torsion box and end cap [impossible to shift these parts if they are solid and present - so work off of them on a rusty truck]
b. Then I would loosely bolt Part B in based on the first picture and then using that....
c. You can build out area C which is actually the B pillar area.

When you have everything mocked up then you can take it all apart, perform rust prevention and then start welding things in place.

To answer your question about part E. It really depends on how rusty the underside of the truck is.

If the torsion box is intact - you will need to do it prior to the install of the inner rocker [as 1949gmc noted] as it will be difficult to get int there to bolt/weld it in once that is done. I did this in the passenger side of my build since the torsion box was in great shape.

If your torsion box is gone or will need replaced - I did it just before I reinstalled the torsion box - you will have a clear path to install it under the truck with that long box gone - so it really depends on what you are working with. I did this method on the driver side of my truck since the torsion box was pretty bad and I replaced it.

Give us some pix of what you are working with and we can provide more in depth assistance, if needed.

Good luck!