72" is way to long to get the performance from a sinew backing. If your back is sound leave it bare, if it has durability issues back it with rawhide, silk, linen or brown bag paper.
Ash takes stain very well. You could either use a stain full strength or "water" it down to get a washed effect. If you use leather dye or rit dye use alcohol to "water" down the dye.