CHARLIE,
I applaud you for wanting to retain the factory method and look, after shortening the barrel. All too often short cuts are taken that scream: "This barrel has been messed with." Not that you want to hide anything, just want a nice clean factory original looking result. Albeit more effort and craftsmanship is required.
1. Particularly during the 50's and 60's there seems to be little order to how the base was attached, domed pins, flush polished pins or integral base. Even on the same model a year apart or a different barrel length as mentioned above. This was asked of Roy Jinks and he basically admitted there was little rhyme or reason for the differences other than evolution, and therefore also, there's no specific date of change that can be given.
2. Yes, I have done a couple of factory sight re-locations. You'll need to mill or file off the serrations on the rib nice and flat where the replacement sight will be located. Mill or cut a slot/mortise (with Dremel tool using cut off wheels) for the tenon on the bottom of the sight base. The slot isn't going to show so it doesn't have to be perfect.
3. My 19-1 is flush pinned. If the base is pinned onto the rib, you can cut out the piece of rib left on the piece of barrel and use it as a jig. Clamp it carefully to the rib on the end of the cut barrel to mark the two hole locations. Drill thru the original holes to drill new holes or just to mark the locations, remove the "jig", and then drill the two holes.
4. If you want flush pins, you'll need new pins and to reblue the barrel after polishing them flush, also to blue the muzzle. If it's just a shooter of course you can touch up blue the sides of the rib, end of pins and the muzzle. Domed pins for a traditional look can be used and just touch up the muzzle.
NOTE: well fitting flush polished pins will hide any slight inaccuracy in drilling the holes! Notice in your 3rd photo the pin holes are off quite a bit but don't really show unless you look for it. So even the factory holes were not perfect. (this may be one reason the factory quit using domed pins!)
5. If the base is one piece (integral) with the barrel rib, you can cut it out and silver solder on, then reblue the barrel. Or buy a new sight that's pinned on, cut a mortise for the tenon for extra adhesion and silver (hard) solder on. If you're not planning to reblue, you can glue on with Loctite 380 Black Max. I've had success with it on a 45 Colt sight but have not tried it on a 357 Magnum sight. If it doesn't hold you can always go back to the soldering option.
6. Cutting the correct radius on the muzzle crown is important to match the factory muzzle.
My 1950 Target .45 ACP/AR, Pre Mod 26 cut to 4 ¼”, with flush pins. The entire gun needed a reblue:
I hope this helps,