RE: maximum distance front engine bearing to prop centerline?
Boy I was lost on this one till I read it 5 times. I would guess from a hack engineering stand point (My dad and a good friend of mine are engineers) I would be more interested in what the shaft will plug into. The flex shaft was meant to spin a plastic line head and a little piece of string that really provides little resistance (ei even if it hits something, it will break the line with little interuption of power to the head) Mounting the prop hub right on to the flywheel is the way most people convert these. I can see how your installation would be clean, and easy, but I would bet that the propeller's resistance is going to waste either the flex cable, or the points where the cable attaches to the flywheel, or at the prop hub. The flex in the cable alone will cause problems, when you goose the throttle, the cable will load up, and then transfer the torque to the prop, and when you decelerate, the prop's momentum and weight will load the cable the other way. The bearing on the end may serve to center the cable coming out of the tube, but that cable will be flexing in front of and behind it, and possible cause a flutter on the bearing.
My guess,
Good luck.
Better yet, why don't you just get rid of the flex shaft, and attach a long aluminum or titanium bar to the flywheel, maybe a u joint at the prop, so you can get your angle, and Git R Done?