WingWorks, best vest in the business.
12 or 20ga for wild pheasants in SD, good luck bringing down the tough wild birds when they flush at 40yds. Im a big fan of O/U's but the traditionalist will push a SxS. nothing wrong with taking your A400.
Ive been using a Danner 8" Pronghorn with reinforced toe and 200 Thinsulate for my upland hunting for a few years now. Any lace up that you can put some serious miles in over questionable terrain will do.
We will be in SD at Scattergun Lodge in late October for pheasant, let me know if you are in the area at that time.