The usual methods are:
Careful measurement. Really this doesn't need to be that precise, you are not looking for a press-fit between bolts and clearance holes.
If it goes wrong, just elongate a hole into a slot using whatever tools you have to hand (e.g. a round file, clapped-out old Bridgeport, ...)
- Remember: "A
grinderfile/filler and paint make me the weldermachinist I ain't".
Put pointy or painty things into the holes then press the steel plate down in place to mark it. In the dead-tree-carcass world there are things called "dowel pins" used for this kind of thing.