Spindles and the ends of the axle can and have been machined to make them work together, but there are other things to consider. One thing is camber angle. The ends of the axle are built with a certain amount of angle inward to them. If you look at your axle you will see the top of the kingpin is leaned inward. Correspondingly, the kingpin is built so that it also has that angle to it, and when the two are mated the end of the spindle holds the wheel in a straight (or almost straight) up and down position, when viewed from the front. As the wheel is turned there is a particular arc to it that facilitates turning properly.

If you machine another cars spindles to fit the angle that the axle leans and the angle that the spindle leans might be different, so the wheel, when viewed from the front, might lean in or out somewhat. That is something that you might have a problem with.

I do remember seeing somewhere years ago where a guy built a Hupmobile and he had updated it with later brakes, etc. But that was probably in the 70's or thereabouts, so I really don't remember the particulars on how and what he used. Like I mentioned, some early Ford, Chevy, and Dodge vans and pickups used a parallel spring front end. If you hit a truck junkyard with a tape measure you might find one that is a close fit, that way you might get away with little or no changes aside from maybe shifting the spring to spring width or shimming the mounts to get more or less caster.

However, don't let this info I am giving you deter you from seeking other ways of doing it. I've never done a Hup so I am only giving you some possibilities, not absolutes. As for if the single spring setup would be strong enough, yeah, especially if you used one from a maybe 40-48 Ford as they were designed to support similar weighing cars. But they would entail some work there too, building a new crossmember to support the spring and mounts to affix the rear of the wishbones too. All doable but still not a drop in.

Don't get discouraged, you will find a good solution in the end, it will just take some research and trial and error. Anytime you play with a car that is not as common as a Ford or Chevy you take on some additional challenges, but having a different car in the end makes it worthwhile IMO.

You are asking good questions and that is the important thing.

Don