Hi. I looked through my Kinkaid book on SOU rolling stock, and found these cars:
Roof Hatch Equipped 40' Box car:
SOU 26950-26999, 55tonLC; 40'6"; 4037 cu ft.
The picture looks like a modified PS1 boxcar with yellow 8' or 9' Youngstown sliding door. 8 round hatches total on the roof, with similar placement as model above. You could use an old Athearn 40' box as a starting point, but the sill is wrong (prototype one is straight), as well as the car ends (PS ends, not dreadnaughts), and roof (not PS). I'd think a 40' PS1 by Kadee would work nicely, but you'd have to modify the sill. Even though the running board is still in place, the side ladders have been cut down (picture is from 1977).
SOU 32434-32499 70 ton LC; 40'7"; 4037 cuft
A PS1 boxcar with a "fishbelly sill" (think the sill on a Accurail modern 50' exterior post box car). This car as 8 30" round hatches mounted the same as the previous car, with the running board in place as well as chopped ladder (picture is from 1981). Door is similar as above example, in color, type and size. The Kadee PS1 boxcar's sill is wrong for this one.
There were a couple of 50' cars with roof hatches as well.
Roof Hatch Equipped 50' boxcar
SOU 34200-34299 70 ton LC; 50'6"; 4954 cuft
Closest match: Kadee 4105 with 9' Youngstown doors
This car is a 50' PS1 with yellow doors. I'm not sure if it is 9' or 10' door, as it looks to be a 10' door. Running boards are in place (photo from 1979) as well as chopped ladders on the side. 4 total (like the 40' Athearn model above) hatches.
SOU 34300-34424 70 ton LC; 50'6"; 5219 cu ft.
This car is a Pullman Standard boxcar with exterior posts (unlike the previous ones) and a yellow 10' Camel door. It has running boards, but this boxcar was built in 1968, it was likely built without running boards and then added later (1980 at Haynes Shops) along with 4 30" round hatches. I'm not sure if I can find a close model, as most of the exterior post modern 50' boxcars in HO scale are of ACF, FMC, or P&CF design. I hold to the 3' rule, so I'd start with a Accurail boxcar, you have the right sills, right number of posts, and door. The ends of the Accurail boxcar are wrong, and you'd need to add a running board and end ladders along with the hatches.
SOU 551360-551418 70 ton LC; 50'6"; 5000 cuft
This car is different from all the others because it is an Evans double-door car. However, it uses 7' Superior plug doors and 8' door Superior sliding doors. The sill is a lower like a fishbelly sill, and has 4 hatches and a running board with end ladders. The herald (SOUTHERN/GIVES A GREEN LIGHT TO INNOVATIONS) is on the right side of the car as they did with many of the double door boxcars they had. The picture is from 1988, and does not have yellow doors. It has 4 hatches only.
Source: Southern Railway Color Guide to Freight and Passenger Equipment. James Kinkaid. Morning Sun Publishing. 1996. ISBN 1-878887-60-2. Library of Congress Catalog Card Number 95-082160. Pages 40 and 54.
Check here for pictures:
http://www.rr-fallenflags.org/sout/sou26942ach.jpg 40' boxcar from first series
http://www.rr-fallenflags.org/sout/sou26961ald.jpg in number series, but different door and stenciling
http://www.rr-fallenflags.org/sout/sou34255ach.jpg first of the 50' series, but one hatch isn't visible
http://www.rr-fallenflags.org/sout/sou34263akg.jpg a newer shot
http://www.rr-fallenflags.org/sout/sou34291akg.jpg a 1988 shot, with fewer hatches (looks like some have been removed) and painted in the post-merger (NS) paint
http://www.rr-fallenflags.org/sout/sou34301asb.jpg the exterior post prototype
http://www.rr-fallenflags.org/sout/sou34304ach.jpg this one and the following are good for stenciling
http://www.rr-fallenflags.org/sout/sou34304bch.jpg
http://www.rr-fallenflags.org/sout/sou34304cch.jpg
http://www.rr-fallenflags.org/sout/sou34311ach.jpg a new shot
http://www.rr-fallenflags.org/sout/sou34347agd.jpg one with a replacement Superior door
http://www.rr-fallenflags.org/sout/sou551380akg.jpg the double door Evans boxcar
Hope these help. You've inspired me to do one of these as a project.