Not sure where your interests lie, "dinkin" around with "odd/old/obsolete" type calibers (25/20 and .256 winmag) or just being able to quickly load up a bunch of straight walled .38's and go shooting.
I own some stuff in .256 and think it's a great cartridge, mild recoil (hardly any), great velocity & so on. Brass hard to come by but I've got most of what I'll ever need. My question for you is why reline/rechamber for 25/20 (I've got a couple of those too)? Why not just reline and keep the .256 chambering. Brass can be made from .357 (info here on the 'fire) or you can find it at gun shows and online sometimes. Oh, an added benefit, you can run mild loads in the .256 with cast bullets and have your 25/20 fun too.
AS for the conversion to .38 special. If you're going that route why not chamber for .357 magnum (maybe even .357 Max, assuming the Martini action can take .357 Max pressures) as it is already chambered for .256 which runs slightly higher in pressure than .357 if I'm not mistaken. Either of the .357 chamberings would allow you to use 38 special or download their cases to .38 levels and have the flexibility to run full power loads too.
I don't have issues with loading 25/20, but I'm a slow loader type of guy. The .38 would certainly be easier to handload. Personally, I like oddball calibers and the ability to download from full power loads, so I'd stay with .256 or 25/20. I can certainly see utlity in a nice .357/.38 though.
Good luck whichever way you go, sounds like a fun gun.
Geno
