Need a measurement on Lionel 18" Dome Car

I've got the 6-25600 set of Canadian Pacific 18" cars.  Just measured the observation car, and the plastic inset for the dome is exactly 5.75 inches long.

THANK YOU!  I really do appreciate that you took the time to measure and to do so with exactitude.  The opening in the roofs are exactly 6" so the Lionel domes won't be a simple drop-in; however, it shouldn't be too difficult to add .25" of length to the Lionel domes for a cosmetically satisfactory appearance.

The cars are in pristine condition and were made by Clark Benson 30 or so years ago according to info posted here when I asked about what I had.  They'll need trucks and couplers at a minimum; then, there's the whole idea of doing lighting, interiors, detail parts, etc.

So, thank you, again, for the measurement.  With 4 domes comprising 1/2 the consist, having a solution to the dome issue makes this a feasible project.  By the way, each dome car came with a cast lead, two-part dome of great weight and poor detail.  It would have taken a lot of time to make those "right."