For the postal match it's 50 and 15 yds. At last year's muster our targets were a little farther out than what you are saying, but all of our matches at our local cowboy club are. Some of the rifle targets may have been closer to 40 or 50 yds and one was at an unknown distance that was probably around 125. That one had to be attempted, but hits were for bonus. I won't give out this years stages until after the Grand Muster, but if you want last year's e-mail or PM me. As Drydock said about his, our club doesn't really have any set distances for the targets. We just set them out and adjust them until they "look about right". When you set yours up don't stress too much about little details. Make it fun, but challenging. Remember this isn't so much about speed as SASS.
I have shot a match at normal SASS ranges with my .45-70-405 and not had any problems other than a broken weld on a Squirel tree that I shot with a .45-70-500.
The one thing I'd recommend is that your stages tell a true story. Do a little historical research. That's just me you don't have to do it that way, but I find it more interesting that way. You probably can't exactly reinact an historic event for distances and geography, but you can tell the story.