Don't believe Amp makes them for E-350's anymore - unless you find someone to customize brackets for another set they do offer.
I did come across a solution I fancied, but the shop that made these for another customer wasn't interested in making them again (pic below). They incorporated a fold-down step (believe from a handi-cap van of sorts) into the nerf bars.
I hear ya dogtubs. I looked at all sorts of inadequate solutions until I ponied up for the AMPS. My steps were pricey and the guy who did them was using the last parts he had sourced which would fit on a van (he used to work for AMP). They are probably the best spent money on my van though. While the bad news is you may not find anyone who can do it, the good news is my van is for sale ; )