Spindle Replacement
The roller bearings actually never touch the spindles. As they ride on their own inner race that slides on the machined area of the spindles. No way can you tell in a picture if it's OK or not. Have to slide the bearing on and make judgment. And repair as needed or throw away.
Finally! Yes, exactly so.
The actual rollers of the bearings don’t ride on the spindles. And you’d absolutely have to slide the bearing onto that spindle to see if it the inner race has the proper fit on the spindle to be sure.
That said, based on the pictures, I would be pretty skeptical about those spindles being ok. The spindle (and knuckle) are pretty darn tough, so for the spindle to be as damaged as it appears to be took a lot of abuse.
As for 50+ year old, beat to crap spindles of totally unknown history being better than new reproductions from DoctorDiff, not a chance. The spindles the OP showed have been seriously abused. You’ve got no idea what the integrity of those spindles are, and DoctorDiff doesn’t sell junk.
I wouldn’t run those spindles over new reproductions based on the pictures anyway. The bearing surfaces don’t look great, there’s a ton of damage, even the threads are hammered. Even if they’re structurally sound, they’re gonna be a pain. I’d have to see them in person to be absolutely sure, but I think replacing them is the right thing to do.
As for the knuckle/spindle thing, they’re one part on these cars. We aren’t talking about a 4wd for example where the spindle can be unbolted from the knuckle and they’re actually two separate parts. Yeah, technically the upright is the knuckle and the axle stub is the spindle, but you can’t separate them on these cars, so it’s a knuckle AND a spindle because it’s just one part.