I think you might be overly concerned with the top heavy aspect, the big box makes it look scarier than it is. I did a little homework when I got an M1079 a while ago. In retrospect, for the versatility I wish I bought the M1078 and put a shelter on it.
The data plate on the M1078 says the center of gravity (CG) is 47" above ground empty and 55" loaded.
The data plate on the M1079 says the center of gravity (CG) is 49" above ground empty and 56" loaded. (obviously both would depend somewhat on load)
Both are rated for 30% side slope (equivalent to about a 17 degree angle) which is allowing for a comfortable safety margin before the actual tip over point. The 30% side slope is a pretty common rating- some examples: M921, M939, MK48, HEMTT are all rated @ 30%.
The S280 shelter on an M1078 flatbed would have a lower CG than the M1079 because it weighs a lot less. The 12 foot S280 weighs 1,400 pounds and the TM says the box of the M1079 weighs 3,200. I have no idea why there is such a big difference. If you put an S280 on the frame without the bed the CG would only be a little higher than a stock M1078.
Anyway, since I'd done the homework I thought I'd post it. Hope it helps make some decisions easier.