I think that can be worked around (with another block on the side perhaps, I have quite a few of single blocks laying around). I have to think about this for a bit.
6:1 (two triples) has to be reeved a certain way--done in such a manner that the two blocks end-up 90 degrees to one another. Otherwise it won't run cleanly, I'm told. One of the two blocks must have a becket.
Look at the 6:1 reeved right angle here
, on Harken's site. (Somewhere I think they have a bigger, more detailed, image, but I can't find it, right off.)