Hmm, I really don't get where you guys are seeing uphill build. There are very very few quarter horses that have uphill builds, more that are truly level, and most have a slight to severe downhill build.
If you want to see an uphill built horse, look at a good warmblood that is bred for dressage. That is uphill.
I mostly agree with Blue Eyed Pony about her basic conformation... straight hocks, just a weak looking hindquarter in general, slightly back at the knee, downhill (not severely, but downhill nonetheless), long-ish back, very high tail set. Also, her left knee seems to be a bit offset.
However, I disagree that she couldn't have been successful as a barrel racer. I've seen some very good ones with a lot worse conformation than her. The biggest problem with using a less-than-ideal conformed horse for a high impact/high stress sport is that the wear on their body is much more than it is on a horse conformed for the sport.
I really don't see any reason why you couldn't train her for reining. She'll likely never be capable of higher levels but for lower level, local shows, she'd probably be just fine. Plus, every horse can benefit from reining training.