How to apply GD&T to a crosshead guide in a plunger pump housing

I’m starting to learn how GD&T works, but I’ve run into an issue.

For a plunger pump, I’m making this 2D drawing. I set datum A as the surface the housing rests on, datum B as the axis of the large gears, and datum C as the cover surface with the roller bearing for the shaft carrying the small gear.

Now the problem is the area where the crosshead runs. I’m not sure how to dimension this correctly. Should I use a position tolerance relative to B–A–C, or is it better to use parallelism and perpendicularity? Are there other correct methods?

Below are two screenshots showing where my datums are located and what the pump looks like in section.