willshak writes:
Here's where a circular or table saw comes in.
Determine whether you are going to have the studs 16" or 24" on
center. Mark out the locations of the studs on the plates. Make sure
you don't put a stud right where that pipe is.
Measure your stud lengths to fit between the top and bottom
plates. Cut to length, but leave a hair extra so the stud keeps
pressure on the plates. On each stud, measure in 1-1/2" from the front
at the bottom and then another 1-1/2" mark at the height of the
concrete shelf.
You are going to cut a notch out of the stud so that it is only 1-1/2"
thick at the base up to the concrete shelf and the part above the
shelf will remain 3-1/2" and will sit on the shelf.

Why not just mount the 2x4's sideways so that you only have 1-1/2" of
depth the whole way. This should be sturdier and much easier than
measuring and notching out half the depth of a 2x4.