So it sounds like a long shot successfully salvaging a front sight off a stub.

I think bronze-brazing as done by SA holds pretty good.

If I were trying to recycle a Krag front-sight base, I'd be inclined to cut it out of the barrel and file or grind off surrounding metal.

I do not believe Criterion makes a Krag front-sight base.

I saved pictures of some repro-sight examples, I have seen over the years.
I believe both of these photos show an S&S Firearms replica base attached to a barrel.

I am not certain of the origin of the first barrel. It appears 'dove-tailed' and maybe poorly brazed or soldered.

The second photo is a Criterion barrel. Criterion seems to leave a 'flat' to locate the front-sight in relation to the rear-sight holes.
I'm not certain if the base is dovetailed, as well as, brazed or silver soldered.
The owner had to send it back for a re-do, since the base fell off, the first time.

It seems to me, more could be done with the S&S base/flange to blend it in with the barrel contour?

FYI - I also have this question posted on the CMP site:

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As a matter of fact, I started it over there and was advised to post it on this forum.  I did receive a good answer on the CMP forum.  If you take a look, it is at the current end of the thread.

I just picked up sporter 456713 from a local gunsmith after having the barrel shortened  and d/t for a regular dovetailed sight base as the solution for the overturned barrel/crooked front sight. I saved the Criterion stub with the S&S sight base attached that is in the picture BR posted above. I'll take a few close up pics of it then I'll  try to remove it. Curious how effective the installation was after having the first attempt fail.

Criterion mills a flat to locate the front sight. It has a very slight dovetail and appears to be there only to guide placement while attaching. Holding it up  to a light there is a very small gap between the front and rear of the base and dovetail and the dovetail doesn't blend completely with the base edges either. Someone in the CMP link the OP posted said you have to be careful with l/r position when attaching the base but I don't see any measurable excess flat on either side to have to center it between. The S&S base appears to very closely match the flat width...at least on this example.   

I will get pictures up asap - going to the range to try out the new sight configuration.

Why go away for a holiday weekend when you can stay home and look at Krag sight pictures?

Here are a couple backgrounds and some different angles of the S&S base on a Criterion barrel stub.

So it appears the SS front sight is a loose fit in the Criterion dovetail - which might not really be a true dovetail in the first place.  But instead just a flat for "positioning" the sight.

FWIW - It appears the silver-solder joint was strong enough for our purposes.

IMHO - The S&S front-sight base would give a better appearance if the 'flange and dove-tail' idea was dispensed with.
The bottom of the sight base could be ground 'concave' to match the radius of the Krag barrel, near the muzzle.

This would facilitate placement for soft or silver-soldering, give more bonding surface, and better replicate original appearance.