Thanks Carl and others for the interest and likes. Disclamer! 
My explanation is definitley not a teaching moment, just how I did it. This is the second kitbash of a K-line headend car. I followed the modeling suggestions and youtubes of the really talented folks and added some rather inexpensive smaller tools to my collection to tackle these projects. I studied about working with ABS and styrene and adhesives ( I used Playstruct plastic weld) and learned by a few mistakes, especially sanding. I am a poor sander.
Tools: I bought an inexpensive exacto kit that included some saw blades and number 11 razors and some cheap small files with different shapes. The exacto handle has worked fairly well but I tightened it too much recently and it would not hold a blade. So I added a little plastic around the pinching arms and that was enough to hold the blades. But a good handle is important. For making rough or aggressive cuts on the car and thicker styrene I used an exacto saw blade. I tried using the little jig saw blades I had but some of them were too thick and the teeth on some had a wave pattern. For cuts near the edges, I like holding the saw blade in my fingers for more control.
Rivet removal: I used a utility blade on a shallow angle to gently shovel the verticle rivets from the car side. Removing the higher lateral rivet line took more effort and careful scraping to get rid of the shadow rivet line. I used the exacto trying not to gouge the car side.
Marking for cuts and splices: I studied lots of PRR drawings and photos of the express, baggage, RPO and baggage/mail cars. Although the K-line car is a bit short (half inch) it is very close for width and height I think. I made a 1:48 scale drawing of my k-line rpo donor car including existing door and window placement. From the PRR drawings I calculated the dimensions of the doors and windows and placement in 1:48 scale and drew those items on my scale drawing for reference and the measurements for spaces between windows so can see what to cut and where to fill existing openings. I determined the most efficient end of the car to locate the postal doors and windows vs. the baggage door modification.
Filling the windows and adding material to the door openings: Glued thin styrene backing on the inside areas and from the outside, added another thin piece and a 1/16 inch piece to fill in with enough styrene to get as flush as possible. I was able to get it very even. I used the thicker pieces on the outside so if sanding was needed I would have enough to work with.
Large door opening: I narrowed the large door opening per my scale drawing and measurements of the donor car and the scaled down measurements from the PRR drawing. (BM70d)
Postal doors: Most challenging part. The postal door opening was the referrence for the eventual location of the windows. I penciled the exact location for the postal doors on the car. The location was partially over an existing window opening so that is where I made several gentle cuts north, south, east and west toward the perimeter with the saw blade. Then I used exacto blade to carefully and gently carve away material until I got as close as I dared to the outlines. Then I used files, sand paper to and sometimes the exacto to get the edges as clean as possible.
Postal windows: After verifying the accuracy of the postal door opening dimensions, I lined out the space between the door and the adjacent edge of the closest window. Only a small piece was removed from the k-line door edge for that. The far edge of the first window was in the middle of the small K-line door opening so I cut out the exact size spacer to insert between the first and second window edges and glued that in place. I will work on the second and third windows hopefully today.
Filling seams: This was a big challenge on my first kitbash attempt with so so results. I need to make small sanding blocks this time to do a better job. I used fast drying "white putty" I bought at a hobby store. I need to let it cure longer before sanding.
So that's where I am so far. Glad you asked me to explain Carl. In order to photo the tools and other stuff I had to clean up the mess on my project table and found stuff I had lost! Cheers.
While I'm waiting for scalecoat Tuscan Red paint from Minuteman to spray paint my Kitbashed K-line baggage car, I'm starting the next K-line bash to get closer to an accurate PRR BM70d car. The K-line car is short of accurate length by a half inch and the trucks are off about a quarter inch from each end according to my measurements. It's going to be a big challenge for me that involves filling in most of the windows on the stock shell and cutting repositioned openings for postal doors and windows on the postal end and adding 5/16 inch strips to both edges of the baggage opening. Here's a before photo.
