I would stand sideways if I could still get a good shot off, smaller target silhouette and a shot through the arm first before it enters the vitals would be my preference! Perhaps the arm would deflect the round!!
if you're relying on your arms to protect your vitals, not only will you only have one remaining arm to administer first aid on not only your other arm, but also your potentially fatal chest wound...another reason why that wouldn't work, is if your bring your sights up your arms are no longer "protecting" your sides anyways, if that were the case, you'd be shooting from the hip.
if you're wearing plates, you stand square, if not, i would still stand square as a single hit can only immobilize one half (leg or arm) and a center of mass hit would still allow you use of both arms and legs. i wouldn't rely on my arm bone to deflect a round...i'd take my chances of survival by offering more of a target. it's a trade off, standing square increases the liklihood of getting hit slightly, but decreases the potential damage of a single hit...obviously that depends on where you're hit, but that variable is present in any stance...
but in reality, if you
hear the gun shot and you haven't been hit, then you should probably and hit dirt, or take cover...preferrably both...and you would return fire from as concealed or protective of a location as possible. so at the range, whatever is comfortable. i don't think it has a whole lot to do with length of pull.
way off topic, but i would think the length of pull is shorter for the same reason modern assault rifles have adjustable butt-stocks and shorter barrels, it just allows you to manoeuvre better in confined conditions. makes it a better well rounded battle rifle. another logical reason is that because one size fits all would be geared toward a happy medium between short people and tall people, because adjustable LOP wasn't an option, so shorter people can keep the center of balance as close to the body as possible, and tall people will just manage...