If the capes are cut plenty long, you can cut a piece out from the edge of the back or way down on the brisket. I'd do the antelope first, they usually don't hold up so well because they are so thin. Take that strip or chunk and toss it in some water with a little bit of salt. Soak it for 30 minutes and then take it out and tug on it. If it comes apart like a wet paper towel, it's no good. If it holds together and takes the tugging/stretching, there's a chance it's good.
Most tanneries use a alum based tannage which doesn't have much of a shelf life. But I've seen some skins make it without problems. On the other end, a tannery that uses a synthetic tannage will have a better shelf life, if they do it right. I've seen capes tanned with synthetic fall apart after only 2 years too.
The voices in my head may not be real, but they have
some good ideas!