Schuyler , the diagonal paneled roof was in place by the early 1950's. If Speedwitch is unable to find a roof for you, this is what I did to create his pattern assuming you accept Fenton's offer of the X29 roof. I should add the LNE body is the ARA version not the PRR.
—Measure the X29 roof width as this is the width of the replacement roof too. Also measure the half width to guide how much you will narrow each half of the new roof.
—narrow and shorten the X29 roof until it will just fit into the body and sand off the roof details. This will serve as the base for the new roof
—I used the Branchline Diagonal Panel roof but the InterMountain roof will also serve
—Cut the Diagonal Panel roof in half and use the measurements recorded previously to narrow each half
—Dry fit to the new base you created from the original roof and measure width. Use roof ribs to adjust alignment of the panels
—Once happy with the width decide whether to glue roof halves and then reduce the length or glue the halves down and shorten the roof.
—once happy with the new roof each end needs to be detailed to resemble the original roof and new roof saddles for the Running Board.
I hope this makes sense.