Q: Best way to image multiple computers in El Capitan

There is probably a better way but I'm not familiar. You could take one fast external drive and install el Capitan to it then during setup use Migration from the Mac Pro to the external. Now you have 99.9% exact copy except for some minor issues such as a few system preference pane. Then you can boot that external on that Mac Pro and check it out to your satisfaction. Then use it for all the other Mac Pros. You only need to get the install started and don't need to babysit too much.

Maybe:

how to clone 40 mac mini at the same time using network clone (like Ghost)