8 years is a pretty good spread for passenger compared to some and if you model Colorado/ CS/ Denver then a welcome addition. Don't forget they were headed by a D&RGW 'Fx' of some kind going to Denver. More Colorado love 
d
Not to rain on your parade, but I checked my MPHS Eagle archives and Greg Stout's Route of the Eagles, and the 8-1-3 heavyweight sleepers were only used from 1958 to 1960. After 1960 they used 6-6-4's in the service.
So these cars are good for 2 years in their assigned servce and then maybe another 2 to 4 years in pullman pool service, by 1962 MP had enough lighweight sleeprs to replace heavyweight sleepers on all the assigned trains.
My frustration stems from the fact that MP started painting 10-1-2's and 12-1's in Eagle colors in 1948 and they ran until the early 1960's. I'd be ok with stand in cars for MP's 8-5's, which were some of the last heavyweight sleepers to show up in timetables in the early 1960's.
But if you model anything but the Colorado Eagle from 1958-60, you need to have custom decals made and remove the car number and Thrift T Sleeper lettering on the sides.