Floor panel, photo 1. Not much to say here except I had the sheet metal shop press a shallow "X" to prevent oil-canning. You can see it if you look close. If you have a bead rolling machine, go for it!
Side fillers were next. The obvious fix here is a couple of donor doors. I actually bought 4 donor doors and I'll talk about that later. I found that doors were easy to find in the classifieds of my local paper. Not so for cabs and running boards! I paid $25 each. Not bad. First step as shown in photo 2 was to make a big paper pattern of the opening and transfer it to the door outside panels. Note the outer door skin sitting on the top of the truck, soon to BE the top, more on that later.
The rear edge of the door is pretty straight and doesn't get cut, line the paper pattern up with it. Several inches gets cut off the front edge of the door including the top window frame clear back to the center of the door. Can't use any of the slanted part of the frame. Photo 2 shows the inside, front cut on the door. Cut this too long then fit it to the front door post after installation. Unfortunately I didn't get a picture of the pattern on the outer skin but it is obvious how it fits when you get there.
Photo 1 shows the brace installed which gave me a very stable and accurate front top profile.
Photo 2 shows one of the two side panels I made to fit between the bottom side panels of the floor. These two sheets are identical to the ones the factory put in the door frames on the AD pickup at the factory. Study how that floor is made and that is exactly how I made the new rear floor.
Figure 3 shows the brace I welded between the two side panels to support the weld joint in the rear of the new floor panel.
Next I addressed the top cut. Photo 1 shows that the rear of the cut is pretty stable, being supported by a factory installed panel just inches away. The front cut was not so lucky and was very floppy and did not retain its shape. Photo 2 shows paper that I traced the accurate cut profile from the rear of the cab. I transferred this profile to a piece of 1/2" plywood and made a 3/4" rectangular tubing brace from the wood pattern. Excuse the fuzziness of photo 3 but you get the idea.
Squaring everything up at all times is important. Tape measure was my best friend. These three photos show some of the measurements I made. The third one is just a sighting down the three vertical posts of the cab to see that they lined up identically on both sides.
Note in photo 2 the two 1/2" square tubes I welded in to lock in the spacing of the roof.
Next, the body was mounted on the frame and marked for cutting. I took a different approach than I think most people take. I determined that the outer door skin has virtually the same shape as the top of the cab. Thus rather than going the normal route of finding a good donor cab for the rear section of the extension, I would use door skins for the top, a flat sheet for the floor and a couple of spare doors for the sides along with the front and back sections of my original cab.
Photos 1 & 2 show the path I determined to be the best to cut. The line going across the top of the cab traverses the highest point of the cab.
Photo 3 shows the body cut and spaced back 2'. No turning back now!