I can offer my experience from designing/building a metal camera to take the RH/10 which is standard on the 2 x 3 Crown. (see my apug thread)
The top and bottom , and the left side you are referring to, were designed as light traps. The right side light trap is on the Graflex RH/10 rollfilm holder as a little ridge. If you try to grind the camera's left light trap off, not only will the camera be butchered, but, in my experience, unless you machine it flat to about 0.002 thou inch ( 0.050 mm) or less, and then add another light trap further out,, it will leak light.
Also you might have to modify the clamps to press the Horseman tighter than the original design.
For you comment about film flatness, I query what lens you are going to use?
And how do you know it is perfectly sharp across a perfectly flat focal plane?
Particularly, on wider apertures ( lower f/-) , I have pondered it is possible the original 90 ~ 105 mm lens designers might have been quite happy with a little bit of belly in the 6 x7 film plane. ( ??)
And how is the lens going to be more parallel to the film, regardless of what better, "superior" back you use?
Here is a measurement I took with vernier caliper of one old RH/10 holder on top/bottom of each pin roller [ inch] 0.206, 0.197, 0.202, 0.202.
Let us assume my shaky hands and eyes got it right, what is there to do anyway with a "warp volume" of 0.005 inch ( 0.127 mm)
Is it going to affect your ability to focus anyway?
Have fun, but i ( as others above) hope you don't butcher your Crown for an ill advised reason you read on some internet crap !