It looks like grinding the metal to the rollers would give another half inch, BUT...The edge of the bellows is in the way. The folds line up with that metal part so removing the edge may not do any good.
The frame numbers line up pretty well with the "autographic" slot. the area where you can write on the film. I covered the regular round window and use that to see the numbers. I covered it with ruby film and painted most of the film so only the area where the numbers show is actually "ruby." This gives me a 6x12 image with a little overlap on the edges. (each image is actually closer to 6x14 but the usable area is 6x12.
I'm using a Kodak Ektar 101. I wouldn't cover the corners of a 4x5 but since the corners are cut off it works fine, with just a slight dropoff at the edges. Your Optar should give much better coverage. The angulon would be nice but seems overly expensive for this project.
The uncoated 170 anastigmat that comes on the 3a is actually pretty good as it is.