Make sure that you have sufficient space between the spool and the top cross piece to accommodate your webbing once the carriage is raised as high as you want it to go. From how it looks in your first picture (zoomed in on the motor and spool), it looks like you may get only 2~4 wraps around the spool before it all jams against the top cross piece.
You may also want to use something other than nuts as spacers between the cross piece and the gear box. I have noticed that the tolerances on the thickness on hardware like nuts and washers can be quite large. The contact area of the nuts is also quite small so you may want to replace them with a thick plate or a stack of fender washers to prevent crushing the top side of your top cross piece.
You will be putting quite a bit of force on two of the four mounting bolts as you raise the carriage, especially when loaded. Over time, this will likely crush the lower side of your cross piece, making the bolts go loose. You may want to make sure your top cross piece is thicker walled material and since it is aluminum, you may want to add steel fender washers or a 1/16 inch thick steel plate to spread the forces out.
It may be better to rotate the gear box on it's axis by 90 degrees then attach it to the front and rear faces of the cross piece with two plates (I am assuming that there are mounting holes on the other side of the gear box too). You can then easily get any spacing you want between the spool and the cross piece. The fasteners would then be in shear rather than in compression (and possibly crushing your tubing).
Lastly, you may want to add a shallow U-shaped brace between the two vertical posts, about half way up, attached to the outside surface of the vertical posts. We used the same (or similar) carriage wheels on our practice robot and the carriage wheels to slip between the tubes when the carriage is loaded. There is only about 3/16 inch of overlap of the carriage wheels on the vertical tubes so the carriage twisting slightly will pry the two tubes apart and allow the carriage wheels to slip in between the tubes. You may want to grab the two tubes and try spreading them to see how rigid your tubes are. At Robot Remix, this past weekend, the Finalist Alliance Leader robot had two long woodworking clamps to prevent this from happening to their robot
