Thanks for the replies, but nobody has answered the question I pose. I know the methods mentioned are correct, and I accept your information with gratitude, but still asking if anyone can answer my question about grinding the faces to match.
Okay, I’ll bite. I trued the jaws on my 8” 3J, they are 2 piece jaws, so I made a loading ring that tied into the bolt holes that secure the top jaws. I then cut the jaws with a TBC insert - with excellent results. However, to your point “could you surface grind them?” Yes, of course you could. If the chuck is that bad, nothing to lose. I don’t know that gang grinding would be the way to go. The jaw position references the load side of the scroll teeth - not the aft end of the jaw. Still, that isn’t a big deal, grind the jaw, reassemble, map the run out and which jaws are ”tall”, disassemble, grind again, keep going until satisfied.
Two features that will be a touch trickier are that strictly speaking you don’t want the jaws cut straight. When a chuck is worn, it will be bell mouthed, part of the reason for the loading ring is to load the face of chuck which cones outwards, if you then cut the jaws it will be slightly “anti-bell mouthed” - ie straight when loaded. Robin Renzetti has a great video on cutting chuck jaws and the deflection of the chuck body. The second issue is that jaw face contact is concave to improve the grip on cylindrical parts - you could manage that by dressing the SG wheel to a bit of curvature.
An alternative method (that works well) is to put a plug really deep in the jaws and crank down hard, then go ahead and cut the jaws. You’ll still need to remove the small untouched land deep in the jaws (either with a ring out near the front of the jaws, work through the hole), or remove the jaws and grind off that little nub. You don’t get as good a “reverse cone” effect, and lose a little of that very in board end, but it will achieve the majority of the benefits. To cut the jaws, whether you grind ‘em or simply cut with a boring bar probably depends on the equipment you have. I used a TBC insert because I don’t have a decent little grinder arrangement (and the TBC boring bar insert cost a whole $8 delivered Ali Express - on a bundled order). TBC was good enough for Robin, so good enough for me.
Let us know how you make out.