I am (coincidentally) working on four IMR CA-3/4 cabooses for my good friend Nate Goodman (Nato), which involves removing the new (and highly inaccurate) cupola running board on the Armour Yellow cabooses, and putting one of the running boards on the cupola of an early Oxide Red (pre-1949) CA-3, which wore the cupola running board for a few months before they were all removed due to safety concerns.
In doing this, it became apparent that some of the cupola roofs weren't glued on very well, and were easily removable for simple repainting. I was also replacing the new IMR non-outside-swing-hangar (and highly inaccurate) leaf-spring caboose trucks with Panamint 3D printed CB&Q wooden beam "Q" trucks (offset pin hole), which are totally accurate for all UP CA, CA-1's CA-3's and CA-4's pre-1955...and at least a quarter of each steel-bodied CA-3 & CA-4 in the 50's through the middle 60's.
On earlier IMR caboose runs, the IMR outside-swing-hangar-trucks are very good for half of the CA-3's and CA-4's after the CA-5's were introduced with these trucks straight from the shops. Half of each earlier class was equipped with the new steel trucks (friction bearing) and the older wooden beam "Q" trucks were transferred to the older CA's and CA-1's, many of which were riding on harsh-riding leaf-spring trucks.
However, for the Monon caboose, the new IMR leaf-spring caboose trucks (which are totally inaccurate for any steel-bodied UP caboose) on this latest IMR caboose run look pretty close to what you want. This is good news for you.
I can say that all of the six IMR CA-3/CA-4 cabooses I've worked on in the past month the cupola roofs came off easily, but I can't say about the other two roof panels as I didn't try to get them off. However, I DID see that they weren't very well glued on in a couple of cases, so I Tenax'ed 'em on and got rid of the air-space between the roof panels and the car sides over the end platforms.
Be aware that the little protruding car body parts that hang over the end platforms on these models are VERY fragile if not glued to the roof panel above them. They will almost certainly break if they're glued well to the roof panel (as they should be) above them.
You might have some luck with inquiring here and at other N-scale forums if anybody has one or two Goldenwest Models CA-3/CA-4 kits, which are what the Centralia Car Shops/IMR cabooses are made from, that they would be willing to part with. This would greatly simplify your efforts in kit-bashing the UP caboose into a Monon caboose. I have more than a dozen of these kits, but they will be used on my 1947-1957 U.P. Ogden to Wahsatch helper district layout. The kits sometimes show up on eBay also. If you want to use the IMR leaf-spring caboose trucks on your model, you're gonna have to buy a "freight car red" IMR CA-3/CA-4 or an "early" Armour Yellow one with the small lettering...and from the latest IMR run.
You could buy some Panamint 3D printed CB&Q trucks (offset) and put those on either the early IMR freight car red or Amour Yellow "early" caboose you have robbed the inaccurate leaf-spring trucks from and sell it on eBay at a nice profit.
Hope this assists you in your caboose project.
Cheerio!
Bob Gilmore