I installed an...ahem..."fake" front PEM on my 98 LeSabre a few days ago. :P
"Fake" here meaning no added material and essentially what Firebuick said; I simply bought a stock front EM cheap off clubGP (that a guy had already begun to "fakely" port), drew a circle around the ports with a Sharpie (matching to a FELPRO exhaust gasket), then ground out the port to the circle with a dremel. I would say I just smoothed the short side radius (bottom), and took off a little on the insides at the top. Took maybe 30 minutes total, probably an hour if the first guy hadn't done half the "fake" work already.
I'm sorry I didn't get any pictures, but just imagine Firebuick'* pic above with the holes in the flange ground out to match the carbon rings.
If you can wait a few days, I will be qualifying this "fake" mod this weekend with the PowrTuner, and I can report what my "fake" front PEM did for volumetric efficiency, KR, and ability to advance spark.
(Butt dyno says "maybe", but nothing to write home about)
I did consider having a front PEM done at W-Body store, but the cost was prohibitive. Thus, my "fake" job.
I also considered a PowerLog, but couldn't justify $140 or so for something even zzp doesn't put a "fake" hp number on. Also, the kids at ClubGP seem to think their used, rusted out old PLog'* are worth their weight in gold.
2004 Buick Lesabre Limited, Gran Touring; Bone Stock
1998 Buick Lesabre Limited, Gran Touring; Mods: Poly sway bushings/endlinks, Tru-Cool trans cooler, Thrasher shift kit (med), PLIM, front PEM; Mods in progress: Alot of stuff, but slowly...