'I have XS400 forks from a disc brake model, as well as a disc, brake lines, and a master cylinder. I am looking for a wheel and information about a wheel only, hence asking if this particular wheel shares the same spindle diameter and thus would be an easy fit.'
No THAT'S unhelpful. Your inaccuracy in posting detail avoided any mention of it. What I gave you was reality, and I filled in much detail you didn't give a thought to. . You can use a nuclear reactor from a battleship in an aircraft with enough effort and money poured into the money pit, that doesn't make it practical.
Your locked-in thinking about axle size only is still too limited even for wheel ONLY, you haven't taken into account the spacing of disc, the diameter of disc or wheel centering issues in fork and why are you even talking about wheel 'only' if a disc is also involved?
And you still have the speedo issue to deal with. Will the cable fit? Will your speedo read right provided the drive ends are even the same? What if your axle is not LONG enough? Does one screw in and the other clamp? Is the axle stepped for it?
'...spindle diameter and thus would be an easy fit.'
Who said that, you? That is a lackluster determination of go or no-go there on a disc setup for sure. The equivalent of putting a 426 Hemi in a 289 Mustang, they both are v-8, why doesn't it fit??? Pretty funny actually.
I'd hate to see you mod internal engine, with your narrow mindedness you will blow up every one you work on.
Discouragement? I have no need of optimism, it breeds the shortsidedness that commonly messes up things in the world. It is also what blindly leads people into brick walls at 100 mph. I deal in pessimism, it allows one to see the problems (the brick wall) others gloss over saying they are 'easy', while I am the guy actually figuring out how to make those things work and stay that way. How I made a living for 35 years both running and repairing 100 foot long printing presses with hundreds of thousands of parts and you keep talking about just one silly measurement.
It's always been hard for those who try to turn everything optimistic to pick up on discouragement often actually being reality. When It finally doesn't work instead of going crazy I for one will likely already have a plan for making it work while the optimist flips out when the unforeseen consequence happens. NOTHING is unforeseen, it's overlooked in the optimism instead.