In summary, the FJ-2 kit's wing appears to have a little too much wing span and about the right wing sweep depending on the exact location of the trailing edge, which needs some cleanup (and maybe thinning) anyway. Both the root and tip chords look a little too big but not as much as the 6-3's wing. All in all, I'd say its well within my tolerance for error.
Unfortunately, a review of the fuselage picture published by Sabrejet on Britmodeller indicates a more significant problem. It is clearly somewhat long by 6 to 8 mm (1/4 inch). In checking the comment of another modeler who has the FJ-2 kit that the fuselage was actually slightly undersized relative to the published length, I noticed that the FJ-2 overall length including the extension of the stabilators aft of fuselage of 37' 7" is identical to the 1/72nd length of the kit fuselage from the tip of the nose to the tip of the fairing above the tail pipe. That may be the cause of the fuselage length error.
21 October Update: I now have all three kits in hand and they are lovely to behold.
The surfaces and panel lines are engraved and petite. All the detail parts such as the pitot look as close to scale as you can probably do in 1/72. I haven't checked the fit except for fuselage halves and the wings but so far, so good. The canopy is injection molded in two pieces and clear.
The FJ-2 fuselage and wing are not the same, correctly, as the FJ-3's. The FJ-3s have a slightly deeper inlet and forward lower fuselage and a different air scoop on the upper fuselage aft of the break. The FJ-3s have a cambered leading-edge wing that has a representation of the 6-3 planform change that differentiates it from the -2 and blue FJ-3s slatted wing, which were similar to the F-86E's and early F's. (Remarkably, the FJ-3 wings have three of the four barricade snaggers—these are teeny things in 1/72—on the leading edge of each wing, missing only the most outboard one.) They also have alternative rudders and horizontal stabilizers with the external ribbing on the trailing edge whereas the FJ-2 kit does not.
Both the FJ-2 and FJ-3 fuselages are a little less 1/4" too long. Theoretically you could take 1/8" out of the aft fuselage (but you can't do it at the break as I had hoped) and about 1/8" off the inlet. I'm for certain going to forget sectioning the aft fuselage. However, in my opinion, the downward curve in the top of the fuselage forward of the windscreen is incorrect (it needs to curve down more) and the bottom of the intake curves a bit forward and shouldn't.
There appears to be enough plastic in both places to get closer to what I think is correct. See http://tailspintopics.blogspot.com/2017/10/sword-172-north-american-fj-23-furies.html for one approach.
The only difference besides decals between the -3 and the -3M kits is that the latter has two Sidewinders and the requisite pylons. Both have the inflight refueling probe.