One of my earlier machines used a heavy gauge extrusion with 2 slots and I used the slot closest to the spindle rather than go into the centre. I think it was slightly crowned across the width as well.
I used a full length steel insert inside the slot (a long thin bar with the edges chamfered) to thread into rather than lots of small t- nuts and this helped pull it flat at least locally.
There probably was an overall angle to the rail but I also had a similar rail on the underside surface which would slope the other way. When these were connected to Y axis assembly it must have preloaded the bearings slightly trying to make them parallel to each other. I didn’t notice any issues and the machine ran for years. You can check on the Hiwin site what misalignment is allowed and compare to your angle.