Well, it's back to the drawing board for the new and supposedly improved cart. I got the belt mounts setup and when I put tension on the belt I noted that the cart didn't roll as freely.
At first I thought it must be from the stepper, but then upon closer inspection I noticed that one of the bearings was loose!
And found that upon further inspection the tension on the belt (which I didn't think was that much) was torquing the cart over causing the stepper / tension pulley assembly to rub on the bridge!