The Grand Trunk (NEL) rostered three Railway Post office Cars #'s 7810-7812. They were built by ACF in 1929, possibly for subsidiary GTW and transferred at a later date.
The Canadian National Historical Society magazine CN lines Volume 9 Issue 1 has a article pertaining to these cars and similar RPO's used on the Central Vermont and Grand Trunk Western. Here is a picture found in Morning's Sun Colour Guide to Northern New England Cars.
I am going to kitbash one of these cars using a Branchline Coach as the base model.
I recently discovered Joe Smith's Blog "Signal Station 199" regarding his efforts in building a New Haven themed layout. He has a set of blog entries showing his techniques to kitbash a similar RPO for the New Haven. Go take a look, but I warn you Joe's modelling and blog are top notch so your going to be there for a while. Here's the link...
Now that your back, here's where I'm at with this project. Follow along as I put what I learned from Joe to good use.
First off I enlarged a drawing provided in the CN lines issue and copied the dimensions over from the soft copy, that were blurry when enlarged.
Next I cut the floor and spliced it back together to give an length over the couplers of 66' - 61/2".
The floor is braced with a sheet of styrene and some square stock.
The second photo shows the floor sections I removed. I also removed the side vestibule doors and walls on either side of the end doors.