The 1904 McClellan had a self adjusting rigging which kept the centerfire rig in the center so to speak.  I have one that I ride on an old mexican horse and it fits him perfectly.  My Hope saddle has what I believe is full quarter horse bars and I think a full rigging with rear cinch.  I believe that most of the Hopes were centerfire rigged back in those days.  However, my problems is not just slipping backwards but it will walk forward to.  I live in the hill country of Texas where there isn't much level gound so this is a problem.  The saddle does fit my big QH good, but with no skirts and a bare rawhide covered tree their is no "stick" to the saddle on top of the saddle pad.  
I do plan on making my own hope saddle for my other horses someday soon so I am really interested in fixing this problem besides of adding skirts.  I just like the look of the thing, though it isn't much for ranch work, when it's on my horses we look we just rode out of the mid 1800's. 
Have a great day, 
Vince.