It sounds like you are on the borderline between two sizes of the standard advice, and the standard would say you might go with either, but would probably say go long, if you aren't growing. Not that I am a fan of the standard advice, since the outsole length and placement does not always reflect your personal anatomy.
Here is a question: Is there a significant amount of space inside the boot in front of your toes? If so, you might actually be better off leaving a mounting gap near the toes, because the length of the outsole doesn't correctly model your anatomy, making it harder for your toes to press down on the front, and it will be somewhat harder for you to roll to the toe pick. (BTW, that completely contradicts the standard advice, which says the forward mounting plate should flush with the front of the outsole.)
If there is a gap between your heel and the back of the boot where it should touch, I think you do want a roughly equivalent amount of space back there. However, space behind your heel inside the boot could be a serious problem in controlling the boot - you probably need to fill that space with foam or something to get better control, regardless of how you size and mount your blade.
BTW, when I picked a blade that was shorter than what I had had by 1/2", it made a huge difference - the tail dragged, and slowed me down a lot. Not sure if 1/4" would be that big a deal, especially if you are between sizes.
For exact mounting position, another criteria, which I believe to be more important, is that you want the sweet spot - the point where the blade changes rocker curvature - to be directly below, or possibly a mm or two in front of, the ball of your foot (where it bends most easily; the exact optimal placement for you might be a matter of experiment). Anywhere else, and it is hard to control what your foot is doing when the blade is on the sweet spot, which is about where you are supposed to do many turns and spins. That might well affect where you should mount the blade(s). (I'm assuming, because you are asking here, that you don't have an expert enough skate tech to move the sweet spot.)
AFAIK, Wilson blades have two sweet spots up front - I'm not sure exactly why, and am not sure exactly how you should line them up - maybe your ball should be halfway in between - so forward turns can use the front sweet spot, and backwards turns can be a little further back?? You might ask your coach, but a lot of coaches wouldn't know. (It's been a long time since I used Wilson blades, and my skate tech of the time messed up a lot of things, so this isn't from personal experience. For all I know they have two sweet spots to try to best cover multiple foot anatomies.)