I made the bases out of 2x4s, and screwed the drawer slides to the sides. Once these bases were in place, I screwed the cabinets to them.
So what didn't work so well? The 2x4's ended up having either a slight twist or warp to them. When I attached the drawer slides to them, they were a obviously off as well and needed to be adjusted. There is not a lot of room under there to do the adjusting, so it was a bit frustrating. I ended up shimming them to get them aligned well enough for smooth operation. Lucky I didn't build the drawers until after I did this, or they would have been slightly too wide.
What would I have done diffferent? I would have glued and screwed 2 pieces of 3/4 plywood together instead of 2x4's. This would have made a much straighter, stable frame for the slides - no shimming required.
One final positive note, making these drawers extra wide (34 inches) was a good idea. My DW can easily large flat stuff.
