Just a thought here. Why not go with 25-20 Winchester center fire?  Brass is easier to come by and dies don't cost a fortune. 
Just my 2 cents worth.

There was an article in The American Rifleman back in the 30s called “A Slow Twist .25-20,” where somebody chambered a .25 Rimfire barrel to centerfire.  (It’s in my files somewhere; can’t remember if it was a Stevens or a Winchester single-shot rifle.)

As I recall, the operation was successful.  This was back in the day when velocity-crazed wildcatters we’re stuffing powder into every old case (as well as forming new ones), and up there on the list was the .25-20 SS, which (in strong rifles) was being souped up and used with the 60-gr JHP to make a sort of scaled-up .22 Hornet.

Your project should go as well.  All the .25 rimfire barrels I’ve had were shot-out horrors, and eventually were relined, but I never saw the 0.251” groove diameter either.

If you load the Ideal 257420, you will have a nice walking-around rifle that should shoot well out to 125 yards or so, and noticeably flatter than the standard 85-gr bullets.  Your 044-1/2, with its skinny barrel, is not going to be a 200-yard benchrest gun anyway; use it the way the factory intended and you’ll do fine.

I’ve tried the souped-up loadings in one of my relined barrels, and they don’t really get you much.  There are other hot .25s, better suited, and it’s hard to find the 60-grain HPs the old ones were built around, anyway.  It’s a great cast bullet caliber; I’d recommend using it as such.

The problem is Trying to figure out end use and expectations.

If you are not planning on shooting bullets over about 70 grains with moderate loads, which is about all the twist will handle, there is no reason to do anything more than convert the block to center fire.

With the 25-10 centerfire cartridge, smokeless powder and a cast 65 grain bullet, you have no problem hitting 15-1600 fps, while keeping a low pressure, low temperature load. You have reasonable accuracy and barrel life.

If you start hot rodding the load with jacketed bullets, you will likely end up with a shot out barrel in less than 1000 rounds. Unless it is a very pristine example, it's probably about halfway there now.

When you look at 25-20 load data, most of it is in the 8-10 grain range, and you back off that a bit due to smaller case capacity raising the pressure. So it fits the 25-10 case very well.

For what it's worth, factory smokeless loads for 25 Stevens Rf used only 4 grains of powder and velocity was about 1200 fps. Brass was the weak spot.

A 044 1/2 barrel is rather lightweight, you might run into issues with the front sight slot if you want to re-bore it to a larger caliber. A 28-30 might be okay? Anything larger might run into the slot for the front sight. A 28-30 is a fun caliber.

with my m-44 that i had converted to 25/20 winchester i load with light charges of unique, sr7625 and like powders. between 3.9 and 4.5 gr. and the lyman 257420 bullet cast fairly soft and many without gas check.   I just looked up some chrono. data and shot 4.5 gr. of 7627 average 1497 fps with  11es. i dont use my chrono. very often as how fast a bullet is going is not what i am looking for. poking holes in paper or knocking off a sage rat

I do apologize; looked through some of my files and couldn’t find the reference.  Don’t have the steam to wade through my four-foot shelf of disintegrating pre-war American Riflemans.  I remember that title and that article, though, if incompletely.

I have or have had,
4, 25RF all 1/17, all .257
     Favorite, 44, 44 1/2

107, 25-21, 1/14

   44, 25-21, 1/12

  044 1/2, 25-20, 1/12

seems I need a 25-25 to round out the 1/4 bores