Unfortunately blocking isn't an option for me... as I mentioned in an earlier reply, the face is even with the cabinet, and my drawers are built to maximize the available width (only space for the 1" I'll need for the slides).
I actually got the slides today from CSH... ordered the 400mm soft slides after speaking with a guy there who guaranteed me they were exactly 400mm (I have exactly 15.75" for the slides). While they definitely look great and feel solid, UNFORTUNATELY... the slides are actually about 3/32... niiiiice.
Maybe this will help someone else in the future... I'm guessing all their slides are oversized like this, maybe they fail to account for the plastic or something.
Thankfully I only have about two drawers where it's a real problem. The closet falls away about an inch in my sons room (it's crazy, scribing the built-in to fit was excrutiating), and the top two cabinets are +1/16. I can scrape away some of the drywall to give those slides a groove to sit in.
On that note, just finished assembling all the drawers... rabbets/dados didn't come out perfect, but a solid job given I did it all on a ryobi table saw. I can say my next set of drawers will be awesome...
Just some final sanding, poly, more sanding, and then I can start installing!
Thanks again to everybody for all the advice and insight, really great and appreciated.