The M-I box car is "generally accurate" for 40'-6", 10'-6" inside height, A.A.R. 50-ton box cars built by American Car & Foundry Co. The cars had 5-5 Dreadnaught Ends, Murphy panel roofs, 10-panel riveted sides, and 6' door openings with Youngstown Steel Doors. The ladders on the models aren't correct and should be slightly taller and with 8 rungs on both the side and end ladders instead of 7 rungs.
Missouri-Illinois 4250-4299, 50 cars, built 11-45, ACF lot 2837
In addition to the Missouri-Illinois cars, other MoPac box cars of this type were also built for M.P., I-GN, and StLB&M by ACF and Pullman-Standard from 1942 to 1946 as follows. 
M.P. 32500-33299, 800 cars, built 3-42, ACF lot 2341
M.P. 34300-34599, 300 cars, built 9-45, ACF lot 2761
I-GN 17850-17999, 150 cars, built 10-45, ACF lot 2761
StLB&M 18000-18249, 250 cars, built 10-45, ACF lot 2761
StLB&M 18250-18449, 200 cars, built 3-46, Pullman lot 5823
I-GN 18800-18999, 200 cars, built 2-46, Pullman lot 5823
Still others had the same features except with longer side sills that extended from bolster to bolster. 
StLB&M 17501-17600, 100 cars, built 5-42, ACF lot 2419
I-GN 17751-17825, 75 cars, built 4-42, ACF lot 2420
M.P. 34113-34262, 150 cars, built 5-42, ACF lot 2419
M.P. 34263-34287, 25 cars, built 4-42, ACF 2420
The MoPac general arrangement drawings for these 10'-6" AAR box cars built from 1942 to 1946 show side and end ladders 18" wide, measured from the center lines of the bolts fastening the rungs/hand holds to the ladder stiles. The rung spacing is 16" except for the bottom two end ladder rungs that were 15 1/8" apart. 
The ladder stiles are 1 3/4" x 1 3/4" x 3/16" angles. So the actual width is 19 3/4". 
The top 6 rungs of the side and end ladders were lined up at the same height. 
Branchline Blueprint Series (now Atlas) box cars included a set of 8-rung ladders having the correct width and 16" rung spacing. These ladders are likely the closest to being prototypically correct than we can expect to find. 
The total number of the above box cars was 2,300. These cars were part in an article on MoPac's 40' all-steel box cars published in the Summer 1995 Eagle by Ed Hawkins.