I have a V500. It came with 35mm holders, do you not have them? I don't think you can get them anymore, I looked briefly because one side of my holder is broken, and I couldn't find any. You may be able to get away with simply placing the negs on the glass and closing the lid, I do this with 5x7 negs and it works well, though that may have to do with the larger size...
As for selectable scan area, the bundled Epsonscan software (which I do believe is still available for download from Epson's site) allows you to select the scan area by unticking the "thumbnails" checkbox at the bottom.
Good luck!