If one wants to kit-bash a particular freight car, there are several ways of going about it. I will present three ways of modeling RBWX 60000-60232 series reefers built in 1952-1953.
Most modelers have their main hobby interest turned to other facets of the hobby like the modeling of steam locomotives, operation, or even collecting prototype modeling magazines. If such a modeler wanted to model one of these RBWX cars, but didn't want to spend a lot of effort on it, he might go about it like this: He would check in train model catalogs to find a 40' plug door box or reefer and would find the AHM "reefer" No. 5274. Finding that these were not lettered for the GN he'd buy one with any road name, remove the lettering, repaint and reletter, and would turn up with something like the BN RBWX 60047.
Perhaps the project might make him more interested in freight cars. He starts giving his model and the prototype second looks; notes that his model has rivets but that the model has welded side seams. The ends of the model has more corrugations than the prototype, and the corrugations are formed differently. The model has narrow bulges in the roof panels while the prototype has wider and diagonally indented roof panel bulges. The prototype roof is overhanging while the model's roof is a "step-down" type.
If the modeler becomes interested in closer adherance to prototype details in his models, he notes more and more discrepancies. The quickie conversion might then be switched closer and closer to the rip track and the modeler will develop a new technique. He will now search the catalogs for cars with parts like ends, roofs, doors, and sides that look like the prototype he intends to model. In this case the Model Die Casting 50' plug door box is a good starting point. It has a slightly overhanging roof with diagonally indented roof panel bulges. The ends are a 4 x 4 dreadnaught pattern and by scraping away the top bulge and fitting a 1" x 3" rib at the top, a very good similarity to the end of the RBWX car results. The side rivet lines seem to be spaced about like on the prototype and even the plug door seems to be the right kind and size.
To make a 40' car, cut out a 10' portion from the center of the MDC body. From this section, cut out the doors including the section of side sill below them. The door stops should be carefully sliced off with a sharp knife and cemented on in new locations (see photos).
New door openings are made in the two 20' end sections to accomodate the salvaged doors. The two car halves are fitted together at a cut through the sixth roof rib (counted from the ends). This rib will be the center rib on the 40' car. The doors are fitted and cemented into place and the joints are strengthened by cementing strips of styrene across them on the inside of the car. The rivets on the side sill are scraped off to provide a deep enough recess to accomodate new side sills made from 7" (about) strips of thin styrene fitted along the whole side. A 3" wide strip is fitted along the top of the new side sill and butting against the lower edge of the side sheets. I used Northeastern 1" x 3" strip wood cemented on edge to the side sill as door guide but the same size strip of styrene would work as well. Plug door wheels cut from scrapped styene were cemented in place. I empha sized the groove under the roof edge by cutting along the groove with a saw. A 6" wide strip of styrene was then cemented in place on the top of the sides and slightly overhanging to act as the top door guide.
The running board was sanded thin to make it look more prototypical.
At this stage the car body was painted. The ends were mineral red at least most of the time. The roofs can probably be mineral red but silver or yellow were more common in later years. The side are reefer yellow.
The underbody was made of an Athearn 40' box car floor weight cemented to a 40' Hi-Cube box car floor (Part No. 12027). The assembly was then turned upside down so the "wood" scribing shows downwards and the metal weight is on top. The Athearn 40' underframe (Part No. 12026) is fastened to the styrene floor by the truck screws after the brake gear has been cut out and moved to the right places. The underbody and trucks are black.
The difference between the 60104 and the 60138 are, except for the lettering, the degree of detailing. On the 60138 all styrene hand grab and ladder rung imitations have been removed and brass wire has been fitted. A wire grab was also fitted to the left of the door opening. The 60138 also has Cal-Scale brake gear with complete brake rigging and piping added. The MDC end tack boards have been scraped off and new ones made from styrene and mounted high on the ends. Tack boards are also fitted to the sides. The steps under the door were formed from narrow strips of sheet brass with the second step cemented in place.
The car can be lettered either like the 60138 (Champ HR-66) with the two horizontal black bands from 1/32" actual size decal stripes (Champ S-10). They can also be drawn in with a good ink pen. Big Sky lettering (Champ HR-69) can also be used. Presumably, these cars may have appeared with serif lettering (Champ HR-21), but I do not have a photo of a 6000 series car in such lettering. All decal sets mentioned lack the word "insulated" and the RBWX reporting marks. Most of the latter can be taken from other words in the decal sets.