I've read that some of the cargo vans had the seat attachment points, but my 2008 DOES NOT have the mounting provisions for passenger seats. I also don't have the factory mounting points for seat belts. Unpleasant surprise for me!
I ended up using L-Track on the floor and I have it setup for folding Tahoe seats at the moment (3 seats with integrated seat belts and easy removal). My L-Track spacing is also set up for possible Sienna seats in the future, which also have integrated seat belts.
2008 E350 RB 6.0 - Bullet Proofed, MG 4x4 Conversion
2009 E350 RB 5.4 - Camburg Kit (Sold)