I did the same swap, a Graflex Optar 135/4.7 replaced by a multicoated Fujinon-W 125/5.6 (55mm filter version). I was able to use the old lens board from the Optar with the Fujinon, and the lens also fit neatly into my Pacemaker Crown Graphic when folded. My camera is the 4x5 version, but the lens should be the same I think.
If you copy is anything like mine I think you will be very happy with the Fujinon. I did some comparisons using a DSLR as a "digital back" to compare sharpness (my Optar shutter isnt quite working so I dont have photos to compare). Sharpness at F16-22 or so is similar as expected, but contrast is much higher throughout the aperture range on the Fujinon. At bigger apertures the sharpness and contrast difference is major, with the Fujinon being tack sharp at F8 (on a 15megapixel sensor which is 1/35th the size of 4x5 film). Any softness with the lens would be due to mis-focusing or diffraction at small apertures. It also keeps sharpness much better to the corners, has a much bigger image circle, flash PC connector and is easier to find filters for.
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