If it's a flat spring it probably isn't one of the 'soft' ones. How did you determine that? Curious.
For that matter, are the cut down ones really untreated? Mine seems to be hanging in there pretty well for a hundred plus seasons- no wear anyhow. Still a .22, so it hasn't seen any dire pressure excursion.

OK, a couple of questions.  First, you say the barrel was "cut back" to 25-20-WCF.  It sounds like you are saying the barrel was set back, threaded and chambered for 25-20 - correct?  If so, why are you saying you want to make a 25-20 WCF out of it - it already is?

Second, how did you determine the action is "soft"?  If it is a flat spring it is not a Winder - they all have coil springs.  Hardening should not be necessary unless you are going to a cartridge well beyond the modest strength of the low wall.  I recall a DeHaas bolt thrust curve that indicated the low wall was was capable of handling 25-20 WCF, 218 Bee and 22 Hornet.  The 257 Magnum was "on the line" of the upper limit.  Based on this it seems you should be fine with a 25-20 WCF without metal treatment.

David,

A couple of more thoughts.  First, regarding the loose barrel fit, I have had good luck wrapping a turn or two of cotton thread around the tenon to get a tighter fit.  Not around the threads, but right at the end where the tenon relief meets the receiver.  Works pretty well for me, for loose fit or over-index issues.   

Second, be sure you know which type of barrel tenon you have - small or large.  I believe most versions with the flat spring like yours have a small tenon.  The Winder version has a large tenons, like the high wall.  Once you know which tenon you have, you can check ebay.  You may find a caliber you would like to try, or a better fitting barrel to line.  I have a low wall Winder and I have picked up several large tenon 22 short barrels that I have lined in 25-20 WCF, 32-20 WCF and 357 Magnum.   

Great little rifle.  Good luck with yours.

Some of the winders will respond decently well to heat-treating as they are approximately 1522 alloy. Mine is ser no 133xxx. If yours happens to be mild steel i.e 1008 then all you can do is case it. I520 with a decent case and quench in oil should be a pretty tough action, but still needs to be respected as a low-wall. 
In my opinion they are among the most graceful of the single shots. I have no idea when the change over to the high Mn steel occurred. Or even then if is was consistently 15xx steel from a given ser no on. 
Anyway enjoy it.