Widows are cut past center and like a stiff shaft. I would think at CX250, 5575, 400 shaft at about that length with a 100 gr. insert and 135 point would be real close. However, carbons are real touchy on length so I would not count on getting the exact length you want without a lot of fussing around cutting down inserts or something.
I find the 50# range bow to be right on the line between the arrow being to light when spined right with 500 shafts and needing a lot of front weight with 400 shafts. The CX helps with shaft weight, but most shafts don't have the higher weight like the GT. Internal weights can be used, but I like the stiffer shafts and high FOC option better than internal weights.