The procedure for building the cars is to create the sides on the bench, using a 0.010" piece of backing to stack the pieces together. The standard sheets of Evergreen are perfect for this. Select a sheet for each car you are doing and mark a left and right side above the stacking area. Mark a line at the top of the stacking area, use this religiously to align the parts you stack, as it will butt directly against the letterboard. If the bottom runs long or short, we can sand it later (I did the same thing on the real 330). The top of the backing is run long, this will eventually glue to the back of the letterboard of the original car body. For now, we stack pieces across the sheet. Don't worry about the windows being covered by the backing, the material is only 10 thousandths, we will trim them out later with an X-Acto. (See basic 330 carbody stacked in photo with backing still in place.)
One carbody will become the roof line and letterboard, the other the parts donor for the sides. When trimming the roof car, cut away below the letterboard. On the donor car, cut away above the edge of the letterboard. The remaining section of letter board on the car side will be filed and sanded to fit while on the roof the same will be done to remove the walls from the bottom of the letterboard. The 2nd class windows are slightly longer than 1st class windows, this is easily accommodated be removing about 6 scale inches from the center of each donor window. This is visible in the photo. For some reason, I cut out the center sash of the 2nd class windows, but it was easy to add in given the pieces can be glued directly to the backing. For the life of me I can't remember what I made the bathroom windows from, but most likely they were pieced together using the top part of the oval windows. Don't discard the extra vestibule doors, as they are the basis for the half windows on the 302 if you're modeling it.
As for fitting the parts up, I would suggest referring to the drawing of the 330 (A more detailed drawing showing the floorplan is available as a PDF. Also available is Jack Burgess' article from the February 1989 issue of Railroad Model Craftsman on how he scratchbuilt this car. Both files require the free Adobe Reader to view). In order to fit the pieces well, use a NWSL True Sander to create perfect edges. I didn't on my model, and that is one of the things I wish I had (and why I'll eventually scratchbuild one). For blank areas substitute Evergreen passenger car siding (approx 2" scale face), don't use car siding as it is 3.5"scale face. Where you can use multiple windows without modification, do so (2nd class section and for most of the 302).
A few words about the 302, the two carbodies I used (shown in picture) required much less modification. You will note that one side only needs the ends fit up, while the other has an oval window in the middle that can be patched over fairly easy. This is why I opted to build both at the same time.