We just installed as well. The composite base measures about 75" from back to front, with the knock out on the front left at 72" from the back corner of the base. The tub frame overhangs several inches on all sides, so if you take your 91x84 tub layout dimensions and shoot for your front left corner, inset a couple inches both ways you'll be right there for a flex or an elbow to take you right in....
Good Luck, you will love it!