Aaaah. Sorry I did not fully understand your tool. You are way ahead of me. Very nice.
I agree with trying to make the groove smaller.
As far as the groove location, all the engineering books tell you approximately where it should go. Cox apparently either knew better, or didn't care at all. I have two cases in my hand right now. One is a cast case and the groove is about where you put yours. I have an extruded case from a product engine and the groove is about 60 degrees forward of the cylinder location. The groove in the cast case is slightly larger than the one in the extruded case. Both are smaller than your first groove. The Cox extruded case groove has machining marks that suggest that they broached the groove. The groove in the extruded case is parallel-sided. The cast case groove has some taper to it. It looks like the depth gets shallower the farther forward it goes.
I would try putting the groove about 45 to 90 degrees after (in crankshaft rotation direction) the cylinder attachment centerline. You want to feed oil into the bearing before the high pressure (load) area, which is going to be opposite the cylinder. (The bottom half of the case if the cylinder is up.) The idea is that the crank rides on a wedge of oil and does not touch metal to metal. If the groove is too big, or is in the wrong place it may act like an oil scraper.
Good progress, Juan. I am very interested to see how it runs.
Dave