Is positive camber on the dfw only so it will contact the guide rail before/instead of the rear wheel on the same side while rail riding? Or is there more to it than that? If you narrow the front slightly is it still advantageous? Or has the narrower ft track made it moot?

so many questions...

The positive camber on the fdw is done for 2 reasons... one the wheel rides on the stiff outer edge of the wheel and two so the contact between the wheel and the rail is at one point on the inside lip of the wheel. Both reduce friction.... In theory you could skip cutting the body back 1/16" because of the pos cant on the dfw and the neg cant on the rear. Those cants would bring the fdw towards the rail and the rear away. I wouldn't because I want more clearance than that.... any small wiggle may cause the rear to hit the rail and I don't want that so I would still cut back the dfw.