Lots of people (myself including) think you should trim the flares up to be flush to the body over a slider. In practice though, the little gap it creates is really minor and not noticeable. I've thought about this 6 ways from Sunday, and without doing a TON of work, you'll make everything worse by trimming that back wall away.
The only way I can come up with to achieve a fully flat flare with a slider that goes under it would be to cut the flange off the flare where the slider goes. You would then need to plastic weld or use some sort of adhesive to reattach the flange to the flare, 3/16" of an inch offset from its previous position. Now you'll have a 3/16" gap at the "top" of the tab...so you need to try and fill that in, or have a very floppy tab. Finally, I believe that after all that work, the tab will still be fragile and break at the slightest bump offroad (or errant shopping cart).