Yep, definately measure your spring distance from center to center and go from there. I am running a van width and my perches are roughly 5 3/4" away from the drum backing plates. Best to put the axle underneath and triple check your measurements and make sure it is center.
First set of perches was slightly off by maybe 1/4" and annoyed me just knowing that. Would also be a good time to set your pinion angle at what you feel best with.