Dave,
That is one of two holes for the back support bracket. The other goes up through the floor pan right next to the hat channel. There is a stud plate that sits in that floor pan hole (shown below) that works with the bolt that goes through the hole in the side of the tub that you want measurements for. I managed to pry up one of my plate studs intact enough to get a replacement made.
There is also a spacer that goes between the step rear support bracket and the side hole to protect the roll at the bottom of the tub (washers may work).
You may already have all this.
The 3B Page has a good article on the steps:
https://cj3b.info/Tech/Steps.htmlthat article includes a link to the 2A forum. That 2A forum thread has detailed drawings of the step pieces (I copied them for future use!) - including the rear tapered support bracket. I'd suggest using those measurements to verify before drilling.
Here's what I got off my tub (keep in mind I replaced the sides with NOS so the measurements may be off a little due to the shop welding everything together):
From the crease (the bend not the seam) I measured 9 1/2" also, from the rear wheel opening forward I measured 10 1/4"
I checked both sides and they are within 1/16" of each other. The holes are 7/16" up from the bottom edge of the tub. If more owners could check to verify that may be best.
I would do a small pilot hole or mark and verify against the bracket drawings to ensure once drilled, they will be close enough to fit the bolts and hold your steps.
Parts Manual (PM) shows 5/16" -24 x 7/8" bolt for the side hole.
HTH