OK, I've had some time to fool around with this jig. I found that if you use the mag release hole (which most castings & forgings have already drilled) to center the blank in the jig, the top plane lines up. (I used a 13/64" drill bit). If your lower doesn't have the top plane leveled off yet, you can mark it and then remove it from the jig to file. If you're milling, you can do it right in the jig, being careful of course...
Initially, you only need to take off the forward 6 5/16" of the top plane (measured from the furthest-forward point), keeping the rear contour where the upper fits intact for now. Then you can remove the jig (keeping lower clamped in place) and drill the rest of your holes.
Removing the metal in the rear contour can be done now, using the front takedown pin hole as a pivot, and fitting the rear contour area slowly (filing or milling). I use a 1 1/2" end mill and plunge-cut the contour, a few thousandths at a time (trial-fitting in between cuts). Can't do it in the jig, as it's not cut out in this area.
The buffer tube hole can be done in the jig; the jig will level the lower and can be clamped to an angle plate on your lathe (if you use a lathe). I use a 1 1/8" end mill to cut the buffer tube hole, then tap it 1 3/16" x 16tpi in place.
There's a couple enhancements I'd add (and probably will, to mine), but it does the job right out of the box.
An excellent alternative to a tedious job!
I know that Tannery Shop rents/sells a steel-bushinged jig for this same job, as does OSI/Savvy Survivor, but for only a few lowers, this should do the trick.