Has anyone tried to install these racks? Should I just try and stretch them out to make them wide enough or should I buy a different rear rack?
Yes, you can stretch a rack, but not by very much. Whether you can stretch these out to fit depends on how much you need to stretch the rack. This rack is made from steel, which flexes a bit, but take care not to stretch it too much or that could put too much stress on the welds.
Another thing to be careful of: With the rack stretched a bit, are the allen screws into the bosses on the bike frame going in properly? If the screws and the threads in the bosses are clean, the screws should go in very smoothly. Be aware that lateral stress on a screw can cause damage to the boss threads in the frame. Since you're screwing into the horse, this is less of a concern, but it's still an issue.
Also, for troublesome screws, try cleaning the threads of the screws, and the threads of the rack bosses. It's amazing how many problems are caused by dirt in the threads. A drop of lube on the threads may also be a good idea.