hi
what most do is weld
a tube between the a and b posts this will stop the body folding on itself are you removeing the body to sort heater chanell rust ? you may need to weld a tube at the top and the bottom between your posts this is possibly the only surefire way to ensure that your doors will fit when you mount it back up to the pan.
is it going back on the same pan or a different one ?
deppending on how poor the structure at the minute is it may be worth considering welding in some bracing across the body from b post to b post as well.
when youve got the body back on the pan bolted up snug and happy grind the braces off smooth and away you go it sounds painfull but its better than having door gaps all over the shop.