With your bleeds set to 1/8", In Imposer, under the Step and Repeat section, if you set your spacing to 0, you'll get the two bleeds abutting, which gives you the 1/4" that doesn't allow the crop marks to print in your 19" of available space.
If you set spacing to -.06 you'll wind up with a 3/16" gutter, but Imposer overlaps one card over the next. So, one card will have 1/8" but the other only 1/16".
If you set spacing to -.125 you'll wind up with the 1/8" spacing you're looking for, but it will only work if your bleeds match, as one card will have no bleeds at all along the bottom edge, as it will be overlapped by the succeeding card.
I usually use the -.06 for 6x11 full bleed cards, you just have to watch your alignment off the printer.